I now have several problems.
1. Starting off, my midi inputs aren't working. What would cause a system-wide shutdown of midi inputs? Different controllers hooked to different USB inputs, and none work. 
2. There is a box in one of the dropdown menus, I believe it's in the VST plugin menu under Utilities, which has a box which says something to the effect like "Allow plugin to behave as a synth". Something like that. It is faded and the accompanying box cannot be checked. No one seems to know why this is or how to fix it.
3. With the exception of loading a simple midi track and attaching a soft synth which produces music, the midi bus that is created between midi and an arp doesn't work.
4. The only way my arp has worked is if I embed midi information within the arp sequencer. 
 
Anyone have any idea what is going on with my Sonar Platinum? 
 
As an aside to scooks, is there anyone else at Cakewalk who might be able to help? I know you dislike multiple posts in the forum on the same thing, but it appears I have multiple problems, or problems arising from the same system issue, which somehow revolves around either that faded synth checkbox or something wrong with my midi inputs. Should I start over with each individual problem addressed on the forum?
 
P.S. I think that some of these problems may have occurred as I followed different pieces of advice offered to me initially.

Cactus Music, you seem to have things figured out, so maybe you can help me. 
 
What would cause all midi inputs to not work in Sonar? Also, what causes the "Use as synth" box to be faded out? What do I do to change this?

Don't know why, but I turned on my Sonar this a.m. and the midi inputs work again.
 
However, the box inside the Utilities>Options>Folder Defaults>Configure as synth is still faded.
 
And the midi instrument>midi track>arp still doesn't work.
 
Also, the midi instrument>arp>synth doesn't work. 

scook, in example #5 you say to add the arp as an instrument track and add the synth as an instrument track. The synth has to be put in an audio track, so when you say "instrument track" do you mean I should set the arp as an audio track and not a synth track? I believe that the arp will still convert to that icon with a keyboard and midi track thingie. 

Maartian, if I am following your diagram your midi signal goes directly into track 2 (a midi channel) and then goes into track 1 (cthulu) and from there to track 4 (rapture session) and then into track 3, another rapture iteration, and then into a midi track labeled wurli (track 8) which then goes into the actual wurli synth (track 7). 
 
Is that correct?
 
Okay, that doesn't seem to work for me. 

Maartian uses eight tracks in order to get his Cthulu to work. That seems a bit over a simple setup. Others have said midi instrument>arp>soft synth. Three tracks. That's nice and simple except that it too doesn't work. Also, it doesn't take into consideration using an arp with a recorded midi track, which is how I would use it. So how about this: recorded midi track>arp>soft synth? Would that, theoretically, work? Not that it has, but the half answers I've gotten so far don't seem to work. And I can't get anyone to address the midi instrument versus recorded midi track. Logically, there shouldn't be a difference. Can someone answer this?
 
Also, is there some setting somewhere within the bowels of Sonar Platinum which affects how inputs and outputs work or don't work?
 
scook, you seem to have some control over this site. Are you an employee of Cakewalk? If so, is there anyone else in the office who might help if, in fact, you have given up helping me?

I would have to say after reading this, that creating tracks in Sonar may not be as straightforward for new users as some other products.  If you have not yet walked through the tutorials in the Sonar documentation, now is a good time.  https://www.cakewalk.com/...t=SONAR&language=3
 
Here is the TL/DR;
 
Sonar has three types of tracks...
1. MIDI
2. Audio
3. Instrument (this is actually a combined MIDI/Audio track)
 
You can split an instrument track into it's separate MIDI/Audio tracks by right clicking on the track in track view and selecting "split instrument track".  That will result in the two tracks becoming visible.  It is worth doing this at least once, so that you can see how the MIDI routing options, and the audio routing options differ.  You can work either way that you prefer.  I find that using an instrument track reduces clutter and makes it easier to insert and use soft synths.
 
A virtual instrument has both components.  It needs to receive MIDI data, and it needs to output audio. The instrument track combines both into one track in the track view.
 
If an arpeggiator plugin can be inserted as a VST instrument, then that is the only thing that needs to output MIDI.  This was discussed earlier and should work as described.
 
The target instrument that you want to play the arp sound with just needs to be setup with the arp instrument as the MIDI input source.  This can be accomplished using either the combined instrument track, or the split track approach.  Any active MIDI source should be visible in the MIDI input dropdown menu of any inserted instrument.
 
If your arp generator VST is inserted as an instrument, you should be able to trigger it with either a MIDI pattern in that track, or an external MIDI controller that is routed to that track.

abacab, you write this: "You can split an instrument track into it's separate MIDI/Audio tracks by right clicking on the track in track view and selecting "split instrument track". "
 
I have a midi track. I try to insert my arp, obelisk, on to an instrument track through the insert tab. It appears in the synth rack with the instrument icon. However, it appears in the track view as a midi track. Right-clicking the track does not offer me an option to split the track. 
 

abacab, okay, I found the split option. I'm sorry, but I am 66 years old and have vision problems (not from looking at eclipses).
 
Midi track. output to arp>. Omni in for arp. Nothing is shown in the arp split midi track, and no sound comes out of the synth track.
 
I know that there must be something wrong with the way I string together everything. What am I missing. I've been working on this for close to a week.
 
Okay, I disconnected the midi track from the arp, muted the arps and redirected the midi to a synth. It still makes no sound. So I muted the midi track, unmuted another midi track and routed it to a synth. No sound and it was never hooked up to the arp. I brought up another song file and everything worked.
 
Any idea what shuts down all midi playback?
 
Actually, my midi inputs and outputs to hardware synths seems to have gotten screwed up when I updated Sonar a few months ago. While fiddling around I rearranged the synths' inputs and outputs, and now none of them seem to work. 

Bob In Portland
abacab, okay, I found the split option. I'm sorry, but I am 66 years old and have vision problems (not from looking at eclipses).
 
Midi track. output to arp>. Omni in for arp. Nothing is shown in the arp split midi track, and no sound comes out of the synth track.
 
I know that there must be something wrong with the way I string together everything. What am I missing. I've been working on this for close to a week.
 

If you have split the track the arp is on, then just use your MIDI controller for input to that MIDI track.  Adjust the settings in the arp to your preferences.
 
Then set up ANOTHER instrument track (not the audio track that the arp is on) with the instrument you want to make the sound with.  Set the MIDI input for that track to be the output from the arp track.  The arp track should send MIDI data to play the sound in the chosen instrument track.
 
Done.