Garritan Libraries will install the necessary components to your Macintosh's Plug-In Components folder (/Library/Audio/Plug-ins/Components).
You can load the ARIA Player to access your library by clicking the Instrument Slot button in the Inspector window.
Choose AU Instruments > Garritan > ARIA Player.
Once the ARIA player is loaded in the Instrument Slot, you can double click the slot to open the ARIA player again, or click and hold on the slot to remove the ARIA player and choose a different plug-in.