You must have an older 802.11b/g model Express. AirPort Utility 5.6 is the last version capable of administering it. That version is compatible with Lion as well as Mountain Lion, however, the installer app will refuse to install it on Mountain Lion.
This dilemma is easily solved though. You simply need to extract the app from the installer package using a package extractor like Pacifist or unpkg. Pacifist is shareware, unpkg is free.
Download Pacifist here: http://www.charlessoft.com/
Download unpkg here: http://www.timdoug.com/unpkg/
Download AirPort Utility 5.6 here: http://support.apple.com/kb/DL1482
When the AirPort Utility .dmg file finishes downloading, open it to expand and reveal the AirPortUtility56.pkg package installer file.
Directions for Pacifist (I don't have screenshots for unpkg but it is similar):
Open Pacifist and from its File menu, navigate to the package and open it.
Click the "reveal triangles" until you see AirPort Utility 5.6.app:
Select it and then click Install from Pacifist's toolbar. It will then be installed in your Utilities folder, and you can quit Pacifist.
Mountain Lion will not allow you to delete its existing version of AirPort Utility. Just ignore it. It and the older version (which will bear the unique name "AirPort Utility 5.6") can peacefully coexist.
If you happen to have an archived copy of the AU 5.6 app somewhere, such as a Time Machine backup or equivalent, you can simply restore it or drag the app to your Mountain Lion system's Utilities folder instead of resorting to the above procedure. It will run without complaint. It is only the package installer that is the problem.
If you have an older computer with a version of AirPort Utility earlier than 6.0, you can administer it that way as well.