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.