An Ethernet connection from one of the LAN<-> ports on the Uverse gateway needs to be established to the WAN "O" port on the AirPort Extreme.
The Extreme needs to be set up to "Create a wireless network" using your choice of wireless network name, security and password. It is critical that the Extreme is set up to operate in Bridge Mode.
My condolences if you are trying to use AirPort Utility in Mountain Lion to configure the AirPort Extreme, as it is not up to the task. Hopefully, you have another Mac there running Lion or Snow Leopard that could be used for the configuration process?
Begin by performing a Factory Default Reset on the AirPort Extreme as follows:
Power off the Extreme by pulling the power plug from the back of the device
Wait a few minutes
Hold in the reset button on the back of the device first, and keep holding it in for an additional 9-10 seconds while you simultaneously plug the power back in to the Extreme.
Release the reset button after the hold period and allow a full minute for the Extreme to restart to a slow, blinking amber light.
Connect an Ethernet cable from one of the LAN <-> ports on the Uverse gateway to the WAN "O" port on the AirPort Extreme.
Temporarily, connect another spare Ethernet cable from your Mac to one of the LAN <-> ports on the Extreme
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the Exteme, then click Manual Setup
Click the Base Station tab below the row of icons
Enter a device name, device password, and adjust Time Zone settings
Click the Wireless tab
Wireless Mode = Create a wireless network
Wireless Network Name = Your choice
Enter a check mark next to Allow this network to be extended
Radio Mode = Automatic or your choice
Wireless Channel = Automatic
Wireless Security = WPA2 Personal or WPA/WPA2 Personal
Wireless Password = Password
Click the Internet icon
Click the Internet Connection tab
Connect Using = Ethernet
Connection Sharing = Off (Bridge Mode)
Click Update at the the lower right
Wait a full minute for a green light on the Extreme
Power off both the Uverse gateway and Extreme
Disconnect the Ethernet cable from the Mac to the LAN port on the Extreme
Wait a minute
Power up the Uverse gateway first and let it run a full minute
Power up the Extreme next the same way
You are done.
I tried all but and got it going, then I had move extreme and situate cords. Then I got two errors when plugged back in (Internet and Dns). Stupid me, I had to unplug. I will keep you posted, but that is it for tonight. I will try again tomorrow night to see if I can resolve. You have any quick tips to get it back up.
Oh, Bob and by the way, until I saw this update, I forgot to tell you your solution was flawless using snow leopard. I really appreciate all your help and persistent replys. I am up and running with all airports working. The only thing I wish is that I could connect them using ethernet vs wireless like I had with comcast, but their up and running and I'm content. One thing I did fisrt was airport extreme, that went ok ethernet of course, then went 2nd gen express (that also connected through ethernet), then I tried another 2nd gen express(that booted me out, and made network unavailbale until I unplugged it. Not a big deal. extreme is ethernet, but all expresses are wirelessly connected. No biggie, but if you know a quick fix, let me know. thanks again, you saved me tons of headache.