Need help with iMovieHD (nsr)

So I had a film I made on 16mm transfered to DVD so that I could work on a new soundtrack. Well I finished the new sound and want to export it in the highest quality possible. I need to give it to an old professor because he is going to do a higher quality frame-by-frame transfer from 16mm to make a high quality DVD of my film, and he needs this shitty DVD just to get an idea of how to sync the sound.

I have done this before with a different version of the soundtrack, and iMovie always exported the video file as nameofmovie.dv

for some reason this time it is exporting it as nameofmovie.mp4, and iDVD is telling me that the film is not of a supported file format..

what the fuck?