1. Install GamePad Companion and, if you are using the xBox 360 drum kit, the Xbox 360 Controller Driver.
2. Now plug your drum kit into your computer. If you are using the xBox kit go to System Preferences and click on the Xbox 360 Controller Driver button to make sure that you computer is seeing you kit.
3. Now start GarageBand and create a new project,select Track > New Track making sure that the Software Instrument radio button is selected and click Create. Choose Drum Kits and select the Rock Kit.
4. Select Window > Musical Typing to bring up GarageBand’s keyboard input.
5. Now open GamePad Companion and tap on one of the drum kit’s pads. The Selected Element drop-down will switch to a numbered button. (Note that these are different depending on the version of kit you have) Now select Single Key from the Action drop-down, and hit the key on your computer keyboard that corresponds to the sound in GarageBand’s keyboard input you would like to play when you hit that pad. Repeat for the rest of the pads and you are done.

Edit: It has come to our attention that people are having trouble getting some versions Mac OS especially Snow Leopard to recognize the drum kit when it is plugged in. The light will flash on but not be recognized by the computer. We have found a solution and posted it below.

1. Disconnect all Xbox 360 controllers and wireless receivers.
2. Download the zip file containing the script and the disk image from here
3. Extract the zip file somewhere.
4. Open a Terminal and run some commands:cd /replace/this/with/the/path/to/contents/of/extracted/zip/file/ #this path probably is Downloads/360ControllerInstall sudo sh 360ControllerInstall.sh
5. Type your administrator password when it asks you for it.
6. Wait for the installer to complete and then wait another three minutes.
7. Connect an Xbox 360 controller.
8. Open the “Xbox 360 Controllers” preference pane in System Preferences and verify that the controller(s) is(/are) recognized.

Click here to view the embedded video.