Click on the “Smart Controls” button in the upper left corner of the GarageBand window. This opens up the “Smart Controls” window at the bottom of the GarageBand window.

 

Click on the “Show Inspector” button at the top left corner of the “Smart Controls” window.

 

In the "Plug-ins section, click on an empty slot to open the plug-ins menu.

 

 At the bottom of the list you will see “Audio Units.” Hold your mouse over “Audio Units” and you will see additional menus open up with plug-in manufacturers and then plug-ins.Select your plug-in from the list.