You can download airport 5.6 here: http://support.apple.com/kb/DL1482 not sure if it works on mountain lion.
Also of use to folks might be this forum post for set up using the Motorola NVG510 modem/router and uverse + airport extreme:
My uverse 2wire is in my basement. Will this work? I want to use the airport express as my wifi internet connection. If I change the password on the 2wire. Connect the ae as a bridge wirelessly and then access the bridge only. Will this work or will I need to have two ae's one hardwired and then use the other one as the main wifi. Let me know your thoughts.
For some reason the DMZ setting on my U-Verse Router won't stick. After changes the settings as recommended above and clicking Save, I get the Successful message. But if I leave the page and come back to it, the settings have reverted. It keeps going back to You have chosen unknown2C9E5F50E169 instead of my Airport Extreme. The second setting which I had changed to Allow all applications (DMZplus mode) has also reverted back to Maximum portection.
Any idea why or how I can fix this?
With the new U-Verse router for the 48 Mb/s service these instructions have changed. (This is for the black Motorola branded U-Verse box). You need to click on the Firewall settings and enter your device password for advanced settings found on the bottom of the box.
Then change the Allocation Mode popup to "Passthrough" as in the screen shot below. And change the Passthrough Mode popup to "DHCPS - fixed".
Finally just select your Airport router from the list and click Save button. Then restart your modem.