Where can I find Audio Units in GarageBand?

Launch GarageBand and open its Settings window. Under the Audio/MIDI section, make sure the Enable Audio Units option is enabled:

If the option was off, restart GarageBand to give it a chance to locate and load all Audio Unit plug-ins on your system.

To use one of the Audio Units you purchased through FxFactory, look for the Plug-ins section for the selected Track, and click one of the empty slots to add a new plug-in:

Scroll to the bottom of the popup menu to Audio Units. Plug-ins are ordered by author. You can access your newly installed plug-ins through the popup menu corresponding to the company that created the plug-in:

If you recently activated your license to a product, and the output still contains an audio watermark, please restart your Mac to have the license information become available to GarageBand.