Yamaha DTXpress/DTXplorer/DTXtreme

Briefly looked over the pdf manual when I got it, and tried to 
fiddle with it, but I get no sounds from it after I connect my 
DTXpress I pad kit to it. Funny is that with both my DTXpress I & 
III drum modules, the triggers play right off without doing anything!
I can access drum sounds from the Alesis DM Pro by pressing 
the 'Preview' button, so it seems to work in this respect. I bought 
it off ebay used.
I know I need to read the pdf manual again, but was wondering if 
someone had a quick answer. I didn't get a paper manual with this 
unit, so I'll have to load the pdf maunual in my DAW machine in my 
studio.

Steve