GarageBand Software Instruments

Hey everybody. I got a MacBook about a month ago. I've been messing around with GarageBand and have a USB MIDI keyboard. I read that you can create software instruments by opening the typing keyboard and dragging sounds in. That works. But I can't find the individual drum kit sounds. For example on the rock kit the bass drum is say "C" and the snare is "D". I want to use a lot of snare and bass so I want my instrument to be C and D is bass and E and F is snare. Where do i find these individual sounds on my computer?

Thanks and I am using 10.6 and the latest GarageBand