You’re not going wrong, Xcode 4 is. This problem is pretty common, unfortunately. Report it at http://bugreport.apple.com/ and hope for the best.
In the meantime you can inspect variables using the console, which appears at the bottom of Xcode 4’s window during debugging. If you have an object “myObj”, you can inspect it in the console by typing “po myObj”. It’s a lot less convenient but it’s better than not getting the information at all.