Yamaha DTXpress/DTXplorer/DTXtreme

I'm trying to use my dtxpress as a midi trigger for a library of drum 
samples using the gigasampler program.  The first kit I tried to use 
was from a company called drag and drop drummer.  The kit samples 
they provided didn't map perfectly to the dtxpress.  The toms and 
cymbals mapped fine, but the kick and snare didn't; there is no sound 
when I hit the kick or snare.  Any suggestions about how to make sure 
instruments match up when using the dtxpress as a midi trigger only?  
(By the way, the DTXpress is in "midi" mode, not "native").

Thanks!