Yamaha DTXpress/DTXplorer/DTXtreme

hi all,

can anyone help me with this?

i found an old MIDI-controller keyboard in the basement of my 
building, and i thought i could maybe use it to access all of the 
cool "GM tone generator" sounds that are listed in the manual (p. 
72).  so, i went out & bought a MIDI cable, plugged the keyboard into 
the back of my DTXpress module, switched the "host select" switch to 
MIDI, & tried to play.  

i was pleased to hear a lovely piano sound coming through my 
headphones, but i can't for the life of me figure out to use this set 
up (keyboard + module) to produce any other sounds... what do i do?  

i have never worked with MIDI before, so any basic assistance would 
be appreciated!

thanks,
scott