1. Create a new track using GarageBand's 'New Track' menu item.

2. Select "iDrum - Default" in the 'Drum Kits' section from the New Track dialog and press 'OK'. (Make sure the 'Software Instrument' tab is selected)

3. Double-click on the track (or click on the 'Info' button) to bring up the Track Info window. Click on the Details disclosure triangle to display the track details.

4. Click on the 'Edit' button (it looks like a pencil) next to the Generator pop-up buttons to open iDrum's musician interface.

5. That's it! Start making some beats!