1. Open GarageBand
2. Double click on the Grand Piano track to open the Track Info window
3. Click the arrow beside Details at the bottom left
4. In the Generator window, scroll down and select Stylus RMX
5. Click on the pencil icon to the right of "Manual"
6. The plug-in will open

Note: Although RMX will work with GarageBand it is not a multitimbral host. Because of this RMX will only work with MIDI channel 1.