My advice is to go to the Box2D forums and check out the iPhone section. There is a post in there on how to set it up to compile in XCode, but you may like to use Cocos2D which apparently has it integrated.
First thing you need to do is get the demo (testbed) running. The code for the iPhone is under Contributions/Platforms/iPhone. It will allow you to see various demo's of the engine's capabilities.