Apple will not let you install AirPort Utility 5.6 on Lion 10.7.4 (or 10.7.5) if you have updated the operating system.
You might want to study a workaround in this thread.
https://discussions.apple.com/message/21397085#21397085
This is not officially supported by Apple, so it may or may not work for you.....if you want to give it a try.