I agree that there should at least be some documentation that tells you how to use the plugin.
Anyway, using it turns out to be pretty straightforward. To decode AC-3 audio, you don't have to do anything at all -- if you have one of the newer Sony MiniDV camcorders that uses AC-3 for instance, you just capture from the DV device.
To output using AC-3 audio, you need to customize the project settings when you go through the "Create Disk" steps. Click on the gear icon at the bottom of the dialog, then click "Change MPEG Settings", and choose "Customize". In the "Compression" tab, you can now choose "Dolby Digital audio" (provided the plugin installed correctly).
You may also want to modify the templates to use the AC-3 plugin if that will be what you'll use most. The "Disk Template Manager" will allow you to do so.