Hi A10 driver. Your old Express is rapidly becoming a relic of the past, but you can still use it for the time being. Use the procedure in this post: How to Configured an old AirPort Express, the text of which follows:
You need AirPort Utility 5.6 to configure an older Express. The problem with that version is that Mountain Lion will refuse to let you install it.
This dilemma is easily solved. 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 do not have screenshots for unpkg but the idea 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.
The most recent version of AirPort Utility included with Mountain Lion should be used to configure 802.11n - capable AirPort Base Stations. Make no attempt to delete it. It and the older version (which will bear the unique name "AirPort Utility 5.6") can peacefully coexist. Use the older version only as necessary for older AirPort Base Stations that cannot be administered any other way.
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.
Keep in mind that Apple's decision to abandon support for your original AirPort Express was for reasons known only to them. Remember that you are using obsolete equipment, should some future OS X feature not work for you.
