Add soft synths track

What is the problem you are having?

To do what you want to do, follow these simple steps...
1. Make sure that the guitar track isn't muted, and that the bass track isn't on solo.
2. Insert the soft synth that you'd like to use to record the bass.
3. Arm the new midi track that inserts into the project when you insert a soft synth.
4. Press record, and record away.

What DAW are you using? And, once again, what is the problem you are having? I'll be able to be more specific, then.

If you just want to play the bass without recording it while hearing the guitar, like if you wanted to compose, you can just hit the play button. If the track with the bass is focused on, when you play it will use that instrument.

If you want to record, you arm the bass track. Make sure the guitar track is disarmed. Hit record. You should be hearing the guitar track while you're recording the bass track.
Note: you can't do any of these things from the Soft Synth window. When the SI Bass opens up, close the window and do all of these things from the main track view.

Let me just add that when I click on the bass strings I see the meter move on the icon that says Bass Guitar 1 Primary OUtput Stereo, but I'm not seeing the meter move on the bass guitar icon. Unless my input/output is incorrect on the midi bass track. Input=Omni, output=SI Bass Guitar.

Sonar7xl. I thought I could just bring up the say, bass guitar and play the notes on the fretboard and record from there.

Adding an instument to a project, in this case, means adding a softsynth. The easy way to do that is to use the SynthRack. When you do this and the Insert SoftSynth Options dialog pops up, select MIDI Source, First Synth Audio Output, and Synth Property Page.
 
There are 3 ways, I know of, to add MIDI data to a MIDI track.

1) Probably the best way (in which your midi data sounds more musical) is to use a MIDI keyboard controller

2) Enter MIDI notes in the Piano Roll View by using your mouse.

3) Using the piano keyboard graphic that is on the Graphical User Interface (GUI) of some softsynth's property page. If there is a piano keyboard on the synth property page GUI of the softsynth that you are using, you can select Enable MIDI Output by right clicking on the instance of the softsynth in the SynthRack and selecting Enable MIDI Output. This will allow you to make mouse clicks on the notes on the piano keyboard, and the sound made will be recorded as MIDI data if you have the MIDI track of the softsynth armed for recording.

In other words, there's no way to do what paulclaro is asking - record what he's playing on the strings of the Studio Instruments Bass?

OK, after Dapper asked one of the same questions as the OP, I went and took a look at the SI product page. One of the SI highlights is:

Use your MIDI controller, computer keyboard, or interactive fretboard to play  
 
This may mean you can use the computer keyboard to record MIDI as you use it to play one of the instuments included with SI. You will have to read the SI help file or manual to find out.