Free plug in from Knolls. Unmult.
http://www.redgiantsoftware.com/product ... mult-free/
I tried this in CS6 and works. Be sure to download the 64bit version.
This plug in makes a black background transparent. The example on the left is using the Set Matte with the channel set to Luminance. The next step would add a Levels & boost the alpha (not shown) to get a cleaner image. The example on the right is one step only of applying the Unmult to the video: