You must be using an older 802.11g version of the AirPort Extreme Base Station (AEBS). If that is the case, you will need AirPort Utility version 5.6 to administer it. Unfortunately this version cannot be installed on a Mac running OS X Mountain Lion ... at least, not in the traditional sense. However, it can be installed using a utility like Pacifist.
Here's another method using Terminal
Open Terminal and issue the following commands in sequence
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"
credits goes to Tyr