Inserting Plugin Instruments in GarageBand

First make sure that Audio Units are Enabled for use in GarageBand: Open a project you're working on or start with an empty projectInsert a software instrument track. It will load an electric piano virtual instrument by default.In order to load an Audio Unit instrument of your choice. open the Smart Controls panel that will appear at the bottomOn that Smart Controls box click on the button that will open the Inspector.Then you will finally be able to access the Plug-ins menu ans load an Audio Unit Instrument... Enjoy!