This is Cakewalk by Bandlab, which is apparently simliar to Sonar, however I see no Forums for the new program.
 
I assign channels to my MIDI tracks, but in MIDI Instruments in Preferences I see Roland, Yamaha, and some drum kits.
 
Are there no other instruments in the program? (i.e. Strings, Analog Pag, Warm Pad, etc.)

You are looking in the wrong place. Here is a video showing how to insert soft synth into MC6. This process is similar in CbB.

 

Yes ^ 
The "Instruments" dropdown has confused many. The names there refer to the presets of actual hardware instruments you may own, not soft synths.

I made a Tutorial of how to use MIDI in any version of Cakewalk. see my signature for the link.