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To install AirPort Utility 5.6 on OS X Lion, follow these steps:
1. Download AirPort Utility from the Support website > http://support.apple.com/kb/DL1482 Don't open the installer after downloading that.
2. Open Terminal and copy:
cd ~/Downloads && pkgutil --expand ./AirPortUtility56.pkg ~/Downloads/AirPortUtility56 && gunzip -c<./AirPortUtility56/AirPortUtility56Lion.pkg/Payload | sudo tar -C / -xvf - "./Applications/Utilities/AirPort Utility 5.6.app"
3. Open Finder, select Go menu > Go to Folder, type /Applications/Utilities and open "AirPort Utility 5.6", so you will be able to use your AirPort on Lion
You don't need to open Terminal to install AirPort Utility 5.6 on Lion 10.7.x. It will install just fine.
Maybe you were thinking about installing 5.6 directly on Mountain Lion 10.8.x?