There is no way that I can think of to use iMovie directly with Dropbox.
It might be possible to get a movie clip from DropBox using the DropBox app, or some other app like GoodReader. Then you might be able to save it to the Photos app. At that point, if the format is compatible with iMovie you could access and use the clip.
The only way to send the Movie back to DropBox would be to use some sort of email interface/service. They do work. However the size of movie files often defeat any attempt to email them.
These difficulties are probably why no one has admitted to using DropBox with iMovie, or given you any suggestions. It's just not a good solution at this time.
Sorry.