It might not be obvious for the uninitiated how to install and use any of these free synthesizer plugins. At least if you are not a professional musician and have no sophisticated music software installed. If you are e.g stuck with GarageBand, Apple seems to have tried to hide installed audio units as much as at all possible. I hope this visual walk-through might help. Most plugins comes with an installer of some sort, so you do not usually need to know exactly where to copy stuff on your system. Let us assume you have already installed the plugin, and are starting from scratch with a new GarageBand project:
1. Start GarageBand, create a new project using e.g a Piano as your instrument. Any software instrument will do.
2. You have some panes on the right side of the screen, the “Browse” pane will be the one initially active, but you will not find your instrument there, although that would have been the only logical place to put it.
3. Instead select the “Edit” pane.
4. Press the popup menu where your current instrument is selected (“Piano” in this case).
5. Find your installed AU modules at the end of that list, and select the synth you want.
6. Your synth is now the active instrument! But how to bring it up??? Press the button to the left of the popup menu, just below where it says “Sound Generator”.
7. Voila! (I have no idea why they have to make it this difficult)