That would explain why you cannot find the "key", since this was one of a number of functions that was left out on AirPort Utility 6.0.
You will need to download and install the much more functional AirPort Utility 5.6 for Mac OS X Lion
Open AirPort Utility 5.6 and click Manual Setup
Go up to the very top of the screen again where the Apple icon is located along with the Help, AirPort Utility, etc menus. Click on the Base Station menu, then click Equivalent Network Password and your "key" will be displayed.
Suggest that you keep both AirPort Utility 5.6 and 6.0 on your Mac.