How do I use GarageBand as a amp/speaker to listen to my Electronic Drum set? I have a MIDI-USB cord already but I can't figure out how to listen to my set through the software using my computer speakers?

If you want to listen to the sounds of your drum set, you should use an audio cable and connect it to the computer's line-in, then create a real instrument track.

If you use a Midi/USB interface, you'll have to create a software instrument track and select one of GB's drumsets as the instrument. Hopefully your drumset's midi notes are mapped to the right sounds in GB.