Have your audio units worked before in GarageBand 10.0.2? Or have you just upgraded to Garageband 10.0? Not all audio units are compatible with GarageBand 10 in Mavericks, see: GarageBand for Mac (v10.0): Unexpected behavior with Audio Units plug-ins
If the audio units are compatible, they should be working as described here: Re: How do I get audio unit instruments to work in new Garageband?
Audio units are installed in /Library/Audio/Components or /Library/Audio/VST.
They are available from the Audio Units brick "AuInstruments" menu, once it is revealed byshowing the smart controls (View menu), clicking the "i" button in the smart controls to reveal the "Inspector" and enlarge the panel, until it it is large enough to show the Audio units.
For example, to play the Zebralette instrument: Click the upper brick in the right column of the Audio Units brick (the lower will add effects, not instruments)