1. Go to Track -> New Tracks... and click the "Software Instrument" at the top.
  2. In the "Instrument" picklist, go to "AU MIDI-controlled Effects" and select Unfiltered Audio G8, and click "Create."
  3. If you haven't already, add a separate audio track and add the audio that you want to be processed by the G8.
  4. Select the audio track, and in the Info pane, click on the Output button and select "No output."
  5. Then in your Instrument track, go into the G8 plugin window, and click the "Sidechain" picklist in the upper right, and select the name of your audio track.
  6. In the G8 plugin window, click the "MIDI" button to enable MIDI, and use the < > arrow buttons to select an appropriate midi note.  The default is C3.

Now the audio in your audio track should be controlled by the MIDI data in your instrument track.