If you are using the SVN (pybox2d >= 2.1), you can look at
http://code.google.com/p/pybox2d/source/browse/trunk/examples/simple_01.py for a simple drawing example. If you are using a previous version (i.e., 2.0.x), you will just have to read and understand the manual (debug drawing) or one of the back-ends.
The short of it is that you loop over the bodies, then loop over each fixture (shape) in that body, apply the body's transform to the specific shape's vertices (in the case of a polygon) or center (in the case of a circle), adjust for MKS units -> pixels (this is explained in the manual countless times), and then draw to the screen with whatever library you are using.