The Retro AS-1 MAS Plug-in must be placed in the 'Plug-ins' folder of the 'MOTU' folder in your Extensions folder. You also must select 'Plug-in' as the audio IO type in the Retro AS-1 control panel.
Now, Digital Performer will recognize the audio outputs of the Retro AS-1 as inputs to audio aux tracks. You can listen to either the main stereo output, or individual MIDI channels output from the Retro AS-1.