Apple, in their ultimate wisdom, have made it impossible to install the AirPort Utility version 5.6 in Mountain Lion. The utility works, mind you, but you can’t install it.

That is, unless you have a little bit of bash-fu on your utility belt, which I do. You can extract the utility from the installer package and run it directly to configure your older-generation AirPort Express. Want to learn how?

Step 1: Download AirPort Utility 5.6 from Apple.

Step 2: Mount the disk image and copy the installer .pkg file somewhere, like your desktop (I will use your desktop as my example here).

Step 3: Expand the contents of the installer package into a folder. Open your terminal and do this:

$ cd Desktop
$ pkgutil --expand AirPortUtility56.pkg AirPortUtility

Then, go into the AirPortUtility folder that you just expanded, navigate into the AirPortUtility56Lion.pkg folder, and run the Payload.

$ cd AirPortUtility
$ cd AirPortUtility56Lion.pkg/
$ open Payload

A progress dialog window will open and Payload will be extracted, which will create a new folder called Payload 2 2. Expand that folder and within it, expand Applications and then Utilities and there you will find AirPort Utility 5.6. You can copy the app into your Applications folder, or wherever you want, or just run it from there if you only need it once.

Congratulations! You can now configure your older AirPort Express (or older Extreme base station) using OS X Mountain Lion.