To start with, you can go to the VIDEO_TS folder on your DVD and copy off the .VOB files to your computers HD. I believe VideoWave will accept .VOB video files.
If your DVD has a menu, and you don't have the original project to start with, then you'll have to recreate that from scratch, I don't believe there's any way to "import" a menu from a DVD into a VideoWave project.
And once you're done, you'll have to re-render the new DVD to include the changes. If you had the original files, and the original project to work from, that would probably be the easiest way to make your changes.
Lastly, since Creator NXT 8 was just recently released, I'd guess you're not running version 14.0, maybe 4?