The manual way - Using Pacifist
Pacifist (CharlesSoft) can be used to extract AirPort Utility.app from the installer .pkg file. This will allow you to run this version and version 6. More information is located here.
The automatic (ish) way - Using an Automator workflow
Several people have made Automator workflows for people who don't want to use Pacifist. One of these is located here.
Why use the older app?
There are 2 reasons why you may want to use 5.6 on ML:
- You use an AirPort Extreme or Express 802.11g for some reason (e.g. music streaming, print server) and need to configure this on your ML box;
- You want the additional features of 5.6 as some features (that I use regularly) aren't available on Utility 6.
AirPort Admin Utility (Graphite & Snow) on Mountain Lion
Doesn't work. Don't bother trying. What I do (if I ever want to set up a Dual Ethernet or Graphite base station) is go to my G5 (running Leopard). If your job involves doing this regularly and you have to use an Intel machine you might want to keep a Leopard partition around for this.
Do I have to do this with Lion?
No. The installer is made for OS X Lion.