Can't hear Midi Drums

Hey everyone! Recently I got Cubase 5, and now Im trying to solve few problems I have.
I tried to record drums on a midi track via Groove agent, The problem is, I cant hear anything… I can see the the cubase recoginzes the notes, but no output…
Midi thru is checked and activated.

I use Lexicon Omega as a soundcard, and it has no problem with instruments… so I guess its a routine problem that relate to the Midi…

Any suggestions?