Yamaha DTXpress/DTXplorer/DTXtreme

HI,
I wanted to increase the number of pad then I link an octopad ( MIDI 
tiggre pad) to the DTXpress module with a midi cable.

Whene I stroke a pad of the octopad the DTXpress play a pianon sound
I did not understood how can I change to get a real drum sound

I tried various nulber of channel , the result is the same except 10 , 
in this case I have no sound

does sompebody already faced this issue ?

Thanks for your help
Regards