Go to System drive>Library>Audio>Plug-ins>Components.
That is strange, this path hasn't been changed in 10.6.4 and is standard for Audio Units. Are you sure you don't see it?You mean System Folder? After that, in 10.6.4 there is no AUDIO folder to go to.
This is a good tip. But - does the latest greatest GPO use ARIA player instead? I'm one version behind. Point being, maybe you're looking for the wrong thing. When you try to load GPO you will find it inside the Kontakt player. If you have Aria, you may have to launch that first, then you see GPO.it now uses the Kontakt Player 2 to work