We install the AU and VST plugins in the user’s Library folder NOT the system Library folder.
The AU plugin is at location:
~/Library/Audio/Plug-Ins/
And the VSTs are at locations:
~/Library/Audio/Plug-Ins/VST
and
~/Library/Audio/Plug-Ins/VST3
The AAX plugin is delivered as a disk image. Double click this to open it, then drag the plugin (on the left) into the folder (on the right). It will then end up at location:
/Library/Application\ Support/Avid/Audio/Plug-Ins
If you need to open these folders use Finder's "Go" menu and choose "Go to folder…" then type in the paths as written above. Alternatively you can use Finder's "Go" and press alt ⌥. This will show you the 'hidden' user library, and you can navigate to the right folder from there.
This is crazy, backwards, and aggravating. EVERY SINGLE INSTRUMENT I buy puts the AU (and everything else) in the Mac HD folder. You should put this in your main instructions in the Spifire Library Manager because it's absurd. At least install it in BOTH folders so your buyers don't go insane!
Thanks for the feedback Adam, I'll speak to our development team about this.