Just saw that there is a new minor update.  Here is what JamOrigin says about it:
 
0.9.5 BETA Released 
Here is another small update. A clever little MIDI Machine can load Scala files and adds Microtonal support (to get it go into "Create MIDI Machines"). Weird issues with Ableton Live 9 were fixed. 
 WHATS NEW: 
* New "Microtonal" MIDI Machine for playing microtonal music. 
* Improved built-in help with index pages. 
* Fixed plugin GUI issues in Ableton Live 9. 
* Fixed missing warning labels for samplerate and buffer size. 
* Fixed a crash bug in MIDI Machine.

Installing X3c this weekend and everything seems to have migrated fine except MidiGuitar which is showing in the utility manager (was able to change to enable as synth) but getting the "missing plugin" notice when starting a file that uses MG.  not sure what's going on there...   ideas?

Try a rescan http://www.cakewalk.com/Support/kb/reader.aspx/2007013315
Starting with X3 there is no need to configure MIDI Guitar as a synth, instead try configuring MIDI Guitar as a tempo-based effect. After inserting MIDI Guitar into the audio track, click the VST2 drop down in the plug-in header and select "Enable MIDI Output"

hmmmm tried the re-scan (MG was already configured as tempo-based effect) with no luck.  even tried deleting the MG dll from the vst folder and scanning and then putting it back in the folder and scanning again, with the same result.  i've checked all the other vst's and this is the only one with an issue.

Does it work in a new project? Not sure what purpose the plug-in serves in an existing project once the MIDI track is recorded.
 
Is it a 32/64bit mix-up? I believe you have been using both in the past. I only use the 64bit version.

well tried a new project...  could only find midi guitar in the "insert" drop down menu under insert soft synth.  set it up.. tested it..  recorded both audio and midi. so i closed it all to redo it to make it a habit and i can't find midi guitar listed anywhere (still in the utility manager tho).  is it only available in the insert menu?  getting soooo close  lol
 

As of X3, it should not be in soft synths at all. With X3 effects can now send and receive MIDI data. Using the "Default All Plug-ins" layouts, MIDI Guitar should be an audio effect in VST2 Folder in the browser and the FX bin menu just like any other VST2 effect. In my case, when I reset and rescanned, MIDI Guitar was properly configured as an effect and shows up everywhere one would expect an effect to appear.
 
It would get more interesting switching between X3 and older versions of SONAR. Then configuring effects as synths would be an issue.

it's been a long computer day for me...  i tried it again.. couldn't find it.. checked the utility manager to make sure it was listed.. checked the effects bin and it shows up.   i appreciate the help mr cook.

jma
Instructions for using this with Sonar (from their website):
 
1. Copy VST pluging to your VST folder.
 
2. Launch Sonar. It should automatically recognize the new plug-in and create needed registry entries (ie you should see the plug-in as an effect in Audio FX menu). If not, use Plug-in Manager to scan new plug-ins.
 
3. Use Cakewalk Plug-in Manager, find MIDI Guitar in Effects section and open "Plug-in Properties". Enable "Configure as synth" option.
 
4. Close Sonar and use Regedit to change generateEvents key from "0" to "1" in HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR Producer\Cakewalk VST X64\Inventory\ -> MIDI Guitar
 
5. Launch Sonar. Add new audio track. Insert MIDI Guitar (now found in Soft Synths menu) to FX bin.
 
6. Go to Synth rack view. Right click grey area of the MIDI Guitar synth block and click "Enable MIDI Output".
 
That's it. Add your favourite synth to a new track and route MIDI input from MIDI Guitar, which should be visible in MIDI input menu.

the part in last line - could I bother you to illustrate routing the midi input ? I'm stuck w/only piano sounds from MG...sorry

If you are using Midi Guitar as a plug-in, set the instrument output to None in the MIDI Guitar UI. Add a synth to SONAR and make sure that input echo is enabled on both the audio track containing MIDI Guitar and the synth.
 
The instructions are a little old. If you are using X3, step 3 configuring the plug-in as a synth is no longer necessary and step 6 is replaced by using the VST2 drop down on the VST header to "Enable MIDI Output".