How to trigger sounds in Garageband with my Roland SPD-8 drum pad

It should be as simple as hooking the midi-out ot the pad to the midi-in of the M-Audio, and selecting one of the Drum Kit Software instruments and then whacking away.

However, your pad might be sending midi notes that are different than the software instrument expects. The Drum Kit is expecting a certain note to play the bass drum. If your pad sends a different note you might here a different sound or nothing at all. As a first step, I would use the grand piano software instrument to make sure that when you hit the pad some note sounds. Then you can work on getting the correct drum to sound when you switch to the drum kit.

This is not something that GB is designed to do, however, there have been several threads here telling you how to create your own sampled keyboard that will play different samples when you play different notes.

Keith Barkley wrote:
how to create your own sampled keyboard that will play different samples when you play different notes.

yes, "build an instrument":

http://www.bulletsandbones.com/GB/Tutorials.html#buildaninstrument