Sometimes you don't need the .dff file but If you're playing on the latest 0.2f dev build then there's a possibility the model you downloaded isn't compatible with it anymore as it was made on a very old version of the game. That's my theory.
So what is this?
Q) When will 0.2f be released? A) It will be released when it's ready and not before, rushing it out is what caused 0.2e to be a failure - However we are shooting for a Spring 2013 release.