Yamaha DTXpress/DTXplorer/DTXtreme

I know that this topic has been brought up in the past, and i've 
tried searching the archives, but it all seems Greek to me.  I'm 
trying tofigure out how to get to the GM Keyboard Voices listed in 
the back of the manual (Guitar, Bass, Piano, etc.), but i can't 
figure out how.  Maybe my brain is too burnt out from work, I don't 
know, but it's starting to get on my nerves.  Is it a MIDI channel 
thing?  I can't figure it out, and this manual is oh-so vague.  Any 
help would be greatly appreciated on this subject.