A lot of the physics has changed in 0.8, and hasnt been updated on the wiki yet. It's due to the new version of box2d I believe. I havn't used it before, but i think that the way shapes are created has change, and the parameters for creating a new circle shape are different now.
Best bet is either to check out the source and see if you can figure it out yourself, or hope someone else knows whats going on