Follow these steps to use MachFive in Garageband:
• Create a new 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 MachFive from the pulldown menu.
• Select Manual and click on the Edit button.
• The MachFive window will pop up.
• From here, you can select one preset (Garageband can not use multichannel instruments).
• To save this as a preset you can call up in Garageband, click on the Save Instrument button.