In case anyone else finds this thread and has my issue, figuered I'd log it as well.
I have an old 1st gen Airport Express (looks like a large Mac power supply) which I use to stream music to my stereo by joining my home wifi network. Recently I "upgraded" to a new Xfinity router and needed to re-set up the APE. BUT -- apparently my APE is too old to be managed by Airport Utility 6.1 (which seems like BS, but so be it).
I followed all the instructions re: downloading Airport Utility 5.6, but upon install was told 5.6 is incompatible with My Mac OS (Mtn Lion 10.8.2). What to do now other than buy a newer APE? Sucky suck suck.
Aha -- just dig out of mothballs or borrow any other older Mac (I happened to use an old silver 12" powerbook) and connect it to the APE to with an ethernet cable. Then use the Airport Setup Assistant software which was already on the old Powerbook to set up the APE. I assume you could probably use some other version of APU as well?Once working and reset (green light), I unplugged the APE from the wall and moved it to where I wanted it (behind my stereo cabinet). All set!
Note that even though the APE and Xfinity wifi router were 10 years apart in terms of dates, I had no problem w/ wifi security protocols. I just used the Xf's recommended setting (WPAWPA2PSK-TKIP/AES) and WPA on the APE. Maybe I got lucky, or maybe that Xf setting is universally compatible? Anyway.
So, net net, what I hope I'm contributing here is what to do if you need to reconfig an old APE and run into trouble installing APU 5.6 on your newer Mac. Best of luck.