I’ve talked about the basics of Box2DFlash v1.4.3 in my Introductory tutorial. I showed how to create your world and how to introduce simple bodies into that world. With the release of Box2DFlash v2.0.0 the basics remain mostly the same, there are a few details changed. I need to go over these small changes before I move on to the next step of adding joints and forces to the bodies.
The first and biggest change (although really a small thing) is that we no longer work in pixels! We now work in units that by default equal 30 pixels to 1 unit (think of it as meters; 30px : 1m). With this change our normal gravity is no longer (0, 300) but instead (1, 10). This makes more sense to me at least because gravity in reality is 9.81 m/s/s. This is going to effect almost everything you do, from applying forces to setting the width and height of a box.