Hi - try the following - if it doesn't work then you might need a new modem.
1. Make sure that the firmware on the modem is up to date.
2. Reset the Extreme to factory mode - to do this, unplug the Extreme - push in and hold the reset button while plugging it back in - wait until the light flashes quickly and release the button - then wait for the light to flash slowly - then unplug the Extreme.
3. Connect an ethernet cable from the modem to the WAN port (circle of dots) of the Extreme - plug in the Extreme and wait for the light to flash slowly.
4. Now use the MPB to reconfigure the Extreme - press the continue button and fill in the requested information - use the following guidelines:
a. Use simple 8 character passwords.
b. Create a wireless network - it can be the same name as your previous network - allow it to be extended.
c. Choose the default security type of WPA2 Personal.
d. Enter a new 8 character password.
e. Choose to share a public IP.
f. Connect via ethernet.
g. Do not allow access over WAN.
h. Accept all other defaults.
5. Update and see if you get a green light - now check and see if you can access the internet - but if it says that you have no DNS servers, choose to ignore and update - you will see the light flashing amber.
6. If the light is flashing, unplug the Extreme - then unplug the modem - now plug in the modem and wait for it to reboot completely - if it has a battery be sure to rmove and repalce it before plugging it back in - now plug in the Extreme and wait for it to reboot - you should get a green light and be able to access the internet.
Report back with your results.
Thank you for such a clear, detailed reply. I thought it did the trick. On the 2nd try, I did a manual setup and made sure I had the settings you indicated. I did the whole reboot thing and the Ignore when it complained about no Internet or DNS servers, etc. I actually got a "Success" message and a solid green light. Unfortunately, what I didn't get was Internet access. Ugh. So, my guess is that it's the outdated modem. Will get a new one and follow your instructions again and hopefully will get online. For now...back to the Linksys!
I'll check back in if the new modem does the trick. Thanks again.
Thanks for the update - I am glad it worked - and if you do update to a new OS such as Mountain Lion, be sure to keep your current AU on your computer - it is more functional than the new AU - you can use the new one to see a diagram of your network and the older one to tweak your settings if necessary.