Follow these steps to use the MOTU Symphonic Instrument in Garageband:

• Create a new software instrument track and then double-click the track in the timeline (any preset will work - but it is advisable to keep it in a similar instrument family).
• In the Track popup window, click on the Details pull-down arrow.
• Under Generator, select MOTU Symphonic Instrument from the first pulldown menu and Manual from the second.
• Click on the Edit button.
• The MOTU Symphonic Instrument window will pop up.
• From here, you can select one preset (Garageband can not use multichannel instruments).
• However, you can layer sounds by assigning a sound to another part in MSI, and assigning them to respond to the same MIDI channel.
  • MSI also has an Expert Mode which gives you control in switching between and layering sounds, even when only responding to one MIDI channel -- check with your MSI documentation for details on Expert mode.
• To save this as a preset you can call up in Garageband, click on the Save Instrument button.

Click here for a PDF version of these instructions that includes screenshots