Can anyone help please

I have a Yamaha DTXtreme 11s with USB and a Powerbook G4 which is running Logic express 7 and for some reason I can use loops and the audio fine BUT I cannot get the Drums to work. I plug them in and logic express sees them and I can play and record them, BUT I can onlt hear them through the phones output on the DTXtreme Unit and out the audio on the mac, I can hear the Audio and sample loops through the speakers but not the drums.

Hope this make sense as i'm a complete newie to this kind of thing

Cheers