As I mentioned previously, Box2DFlash 2.1 is quite different from previous versions of Box2DFlash.
The major differences that I have found so far, are related to how to setup your world and your bodies (bearing in mind that I have only just started with Box2DFlash, and do not know the previous versions).
It took me a while to get my head around the Box2DFlash 2.1 relationship between fixtures, shapes, and bodies.