If you have named both parts with the same file name eg mymovie.mpg and mymovie.ac3 and you have put them both into the same sub folder, DVDA will then grab the AC3 file when you drag the MPG file into your DVDA project.
1. In the project overview window, double-click the asset with the missing audio.
2. Right-drag the AC-3 file from the explorer window to the timeline window and drop on the audio track.
(You can also change the audio file in the properties window.)