This a DVD Movie?
I had the same problem in Nero - The difference is in Nero, could just pick "DVD Video" to make a proper video DVD and it set up everything for me.
I don't actually know what specific settings you would need to set to make a compatible movie DVD, but I know the defaults don't work. They'll make Data DVDs just fine, but not movie ones.
Also, make sure you set the book-type if you're using +R media (-Rs should just 'work').