Starting with a blank project:
1-Go to Project >Add Track > Instrument Track > Garritan Aria Player (stereo)
2-Go to Project >Add Track >MIDI Track - add as many as you want
3-In the Tracks view, assign those MIDI tracks to the ARIA player channel of your choice (Output column > ARIA Player 1-1 for ch.1 ; 1-2 for ch.2 etc.)
4-Load the instruments or ensembles of your choice in the Aria Player Gui , paying attention to the midi channel assigned to each instrument.