Are you trying to add the project files or the resulting MPEG2 DVD compliant files? My feeling is the former as Windows Movie Maker doesn't create MPEG2 (although I'm apparently an "out of date" resource

).
Here's my suggestion. Output from Windows Movie Maker as a DV AVI. Then import this into Pinnacle and create your MPEG2 DVD compliant file from this project. Add the resultant MPEG2 file from this, together with the other MPEG2 file from the other project to Nero Express.
I've got Nero Express on my PC, but I haven't yet used it much (and not to create a DVD), so I'm not 100% sure this will work. I'll sit down and work through Nero Express over the weekend
