Sorry, tried to download your clip three times and it failed to finish every time for me. How big is it? Try DropBox or Yousendit? Or just post a few still images.
Rotoscoping clouds and feathers is an immensely challenging task for even the most advanced compositor. Even for someone very experienced, rotoscoping just a few seconds of challenging footage can take days. This is not going to be easy for you, and the Roto Brush is certainly the wrong tool for the job.
Note that "rotoscoping" does not mean using the Roto Brush tool. The Roto Brush tool is a feature that allows you to in some simple cases avoid rotoscoping. Rotoscoping is the frame-by-frame drawing of masks (or painting) to separate an object in a scene.