It doesn't work that way - Motion dropzones don't become DVDSP dropzones. Think of your Motion projects as your DVD menu directly - put in the content you want IN MOTION, then import the Motion project itself to DVDSP and apply it as a menu.
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press http://www.applemotion.net