The Cakewalk website says (http://www.cakewalk.com/Products/GuitarTracksPro/feature.aspx/Studio-Instruments-Strings) you can use your keyboard to play the virtual instruments included with GT4. Still trying to figure out how to do this. HELP and thanks in advance~!

You need to connect your keyboard to the computer via a MIDI interface.  Once done insert a virtual instrument in your project and make sure the input it receives is from the MIDI interface. 

Thanks for the response. I should have clarified a bit more. While I am looking at getting my hands on a MIDI keyboard, the Cakewalk site (ref. above) mentions that you can use the virtual instruments using your computer keyboard (presumably having certain "notes" tied to certain keystrokes). However, I haven't been able to figure out how to do this.

Thanks again

Ah I see.  I wasn't aware you could use a computer keyboard to play virtual instruments (Never had the need).  I'm not sure how that would work honestly.  The way I would have done it without a MIDI controller is by inputting the notes (and duration) via the piano roll or the staff view. 

There should be button on the control bar that looks like a piano.  When you have your virtual instrument track selected click on it or select piano roll from the menu.  There you will find a grid and you can "draw" the notes in (with your mouse).  You can do something similar in the staff view if you are more comfortable there.  I'm pretty sure there isn't a control bar button for that one and you can only find it in the menu options at the top.

All that said I'm a big fan of using a MIDI controller then using the piano roll to clean up what my fingers can't really play.  You don't need anything fancy, even something with 2 octaves is enough as you can record what you want and then drag the notes up or down an octave or 2 if what you bought doesn't have the range. 

I'm at work so I can't check out the help file, but take a look in there.  I found it was much better than the manual.  Since the bakers went out of their way to make a distinct statement MIDI Controller or computer keyboard, I would think it is possible, but I honestly haven't seen it mentioned anywhere else (but I was never looking for it).

Back when I played Lord of the Rings Online they let you buy different music instruments and then you were able to play them in game via your computer keyboard.  It was kinda fun, but I found it difficult to get use to.  If this feature is anything like that, then I really recommend getting a MIDI controller sooner rather than later because even if you don't know how to play piano it is still much easier than using the computer keyboard.

@thor  If you open Studio Instruments in the standalone modus there is a midi-button. Click on it and now you can use your computer- keyboard to play the instruments. Unfortuanetly it did not work in the Plugin-Modus!