You have to have the full registered legal version, in order to use AC3. Note that the trial, hacked version, etc, do not support AC3. Dolby Digital = AC3 files.
I don't know where you're trying to import the AC3 file. There's honestly no reason to "import" the AC3 at all, into an audio-only area. This is what you do instead:
- Import the M2V video files into the asset manager window on the lower left, in the video area
- Drag it to the "timeline" at the bottom of the screen
- Now double-click it, in the timeline area, to change to edit mode
- On the upper left side are several tabs, pick the audio tab
- Here is where you import AND attach audios to your video. Audios, plural. You can do one track or multiple tracks. Available formats include WAV (uncompressed PCM), MPA or MP2 (MPEG audio), and AC3 Dolby (2ch or 6ch).
I believe this is shown on that guide. (Not watched it in a while, admittedly, however.)