In June 2012, I gave a two hour presentation on iPhone programming, during which I live-coded the beginning of a small sandbox app displaying the capabilities and use patterns of Box2D and Cocos2D, a pair of C++/Obj-C game creation libraries. The source code for the app is available on my github account, github.com/emmett9001/iphone-sandbox
This video is a screencast of that presentation, starting from about five minutes in, when I remembered to start the screen capture. Thanks for watching!
Since it starts from a little after the explanation: download Cocos2D and start the Cocos2d/Box2d template project in XCode. If you make your init method look like it does in the opening screen of this video, you should be fine to follow along.