Garritan does not appear in Virtual Instruments

In the Play Tab:

Have you connected a stave’s endpoint to an ARIA instance?

If the loaded sound is arco strings, wood winds, or brass, is the mod wheel icon in ARIA ‘all the way down’ where it’d be silenced?

For wood winds, brass, and arco strings you’ll usually want to choose a CC1/Mod Wheel dynamics based expression map to begin with. You can add more to it as needed.

For keyboards, mallets, percussion, plucked strings, you’ll want the default/velocity based expression map.

Try doing some searches here in this forum for the Garritan libraries. You should be able to find some good tutorials, and perhaps even some example expression maps to help you get started.

I see where benwiggy shared these very useful documents and expression maps some time ago: