You are trying to use DVDA as an editor. Go to the Project Overview window and delete the old media there. Then drag the MPEG file to the Project Overview menu. If there is an AC3 file by the same name, then it follows automatically, though there's no indication in Project Overview. (You will, however, see the audio waveform in the Timeline window.)
Steve Mann
MannMade Digital Video
http://www.mmdv.com