One of the easiest ways is to use the setup wizard to create a blank score. It should automatically load Garritan instruments in the correct slots. After this score is created, just input some notes to make sure it sounds correct. Then examine the score manager, look at how the instruments are assigned to different midi channels. Look at the Audio/Midi menu and see how the Aria player is setup using the corresponding midi channels that you saw in the score manager.

Thank you, I am beginning to understand now!