The Box2D physics engine for ActionScript 3 is an extremely powerful tool, but can be a little tricky to figure out at first. So, I have put together an example file showing the basics of the engine. The code in the download is fully commented to help explain what exactly the engine is doing, and how you need to talk to it.
It should make learning the basics a lot easier to handle. In the file you will learn to create a Box2D world, static and dynamic objects, and how to render it all out.
I hope you find it useful.