Hum, not sure. I use the free version of it, Fab5, and do not have problems.
But Twilight is a brand new release, so maybe it has some newer/special kind of drm protection?
Have you tried other movies to verify Fab works at all? If it will not work period, sounds like the keys they provided were bad. I suggest you contact the e-mail to see if they can help. It may be a simple fix of issuing you a new set of keys.