Loading DVD2AVI projects
DVD2AVI project files (also known as D2V files), can be opened with AviSynth using a plugin called MPEG2Dec3. Follow the shortcut on the Start Menu in the AviSynth program group that says Download Plugins to open the web page, and scroll down the page to find the plugin. There are 2 different versions of it. Make sure to get the one that says Recommended Version in the description (decodefix100.zip). When you open the zip archive you'll see the MPEG2Dec3 plugin (MPEG2Dec3dg.dll) which you need to copy to the AviSynth plugins directory. Make sure you use the version of DVD2AVI (DVD2AVIdg) that's included in the archive to create the D2V projects.
Once you have the DLL file copied to the plugins directory you're ready to open your project file. Expand the Source category and add the MPEG2Source filter. There are 2 filters listed with that name because there is more than 1 plugin that uses the same filter name, but they're not interchangeable. Add either one to your script and then highlight the line in the editor to find out what plugin it belongs to. The dropdown above the parameters should say MPEG2Dec3::MPEG2Source. If it lists a different plugin you need to click the arrow for the dropdown and select the correct one. The only parameter you need to set is the filename.
Adding audio from a separate file
AviSynth can combine video and audio from separate files. If the audio file is in Wave format, the WavSource Core filter can be used to load the audio file. MPA files can also be loaded using the MPASource plugin. Start by adding the filters to load the video and audio files. Now you need to edit the script directly to add Video= to the beginning of the line that loads the video source and Audio= to the beginning of the line that loads the audio. This creates variables for AudioDub to use. After you have those lines edited you can add the AudioDub filter from the Processing category. Clicking the arrow button next to the video_clip and audio_clip parameters will show you a list of available variables. Select Video for video_clip and Audio for audio_clip.