There's this old discussion on adding Box2D to an existing XCode project:
http://www.cocos2d-iphone.org/forum/topic/1399
As well as that link, I also found these links:
http://blog.zincroe.com/2009/05/how-to-add-box2d-to-an-iphone-xcode-project/
http://gtekna.com/?p=117
But with XCode 4, either the discussions above don't work or one has to compromise and have either "blue" reference folders or an unorganized project folder on disk. After much trial and error I found out the following to be the best way to simply update Box2D within one's project without having to compromise on the way one's project is setup. In other words, you project can look exactly the same as it does now except it has a new updated Box2D instead of an older version.
1. Open a terminal window and download the latest Box2D code somewhere to your disk as described by box2d.org (e.g.):
svn checkout http://box2d.googlecode.com/svn/trunk/ box2d-trunk
2. In Finder, navigate to the "box2d-trunk" directory created above
3. In Finder, create a new directory called "libs", for simplicity put it in the "box2d-trunk" directory.
4. In Finder, copy the "box2d-trunk -> Box2d -> Box2d" directory to your "libs" directory
5. Delete the CMakeLists.txt and Box2DConfig.cmake files from the files just copied to "libs -> Box2D"
6. Open your existing XCode project in XCode 4
7. Depending on which template you used, you'll either have a structure of "cocos2d Sources -> Box2D" or "PROJECTNAME -> Box2D". These are groups which actually map to either "libs -> Box2D" or "PROJECTNAME -> libs -> Box2D" folders on your disk
8. In XCode, right click the "Box2D" group and select Delete. When prompted, delete the contents, not just the references.
9. In Finder, navigate to your project directory
10. In Finder, temporary rename your "libs" directory to something else like "libs2"
11. In XCode, select "File -> Add Files to..." and navigate and select the "libs" folder (the one that contains the latest Box2D) from step 3. Ensure that "Copy items into destination group's folder" is checked and "Create groups for any added folders" is selected. If there are multiple targets in the "Add to targets" box, select the "cocos2d libraries" target.
12. Select OK.
13. By default, the Box2D group will be shown under the root project folder in XCode. Drag this to "cocos2d Sources" or "PROJECTNAME" so it basically looks exactly the same as it did before you deleted it in step 8.
14. In Finder, you'll see XCode created a libs directory with Box2D within it. Drag the Box2D folder from the libs directory to the temporary "libs2" directory created in step 10.
15. In Finder, delete the now empty "libs" directory.
16. In Finder. rename the "libs2" directory back to "libs".
17. Done. Now you have the latest Box2D without having to resort to referenced files or an unorganized folder structure.
There may be other ways, but this way worked for me. Enjoy!
