You will need to configure the AirPort Extreme (AEBS) in Bridge Mode to work with the 2Wire gateway. This means that you will need to connect an Ethernet cable from one of the LAN <-> ports on the 2Wire to the WAN port (circle of dots) icon on the AEBS.
It will really help if you have an additional spare Ethernet cable that you can use to temporarily connect your Mac to one of the LAN <-> ports on the AEBS to help with the configuration. Once the AEBS has been setup, you can disconnect this cable if you don't want to leave it connected.
You need to be aware that the Guest Network option on the AEBS cannot be enabled when the AEBS is configured in Bridge Mode, as it must be to work correctly with the 2Wire gateway.
I assume that you have AirPort Utility 5.5.3 already installed on your Mac, correct?
Post back to confirm and we will begin with the step by step configuration, assuming that you want to configure the AEBS to provide a dual band wireless network.
It is important that you know how to turn off the 2Wire wireless signal once you have the AEBS configured, as two wireless networks in close proximity will likely create interference issues.
Establish the Ethernet connection between the 2Wire and AEBS as described in the previous post.
Connect another spare Ethernet cable from your Mac to one of the LAN <-> ports on the AEBS
Power up the AEBS, wait 25-30 seconds and then open AirPort Utility on your Mac. Select the AEBS and Click Manual Setup
Click the Base Station tab below the row of icons to name the AEBS, set up a device password and adjust Time Zone Settings. No check mark is needed next to Allow Setup over WAN
Click the Wireless tab and adjust settings as follows:
Wireless Mode = Create a wireless network
Wireless Network Name = Your choice
Enter a check mark in the box next to Allow this network to be extended if you expect to add another AirPort router at a later date to extend the network using wireless only. If not, leave this box unchecked.
Radio Mode = Automatic
Channel = Automatic
Wireless Security = WPA2 Personal is preferred, otherwise WPA/WPA2 Personal
Wireless Password = Your wireless network password
Click the Internet icon
Click the Internet Connection tab
Connect Using = Ethernet
Connection Sharing = Off (Bridge Mode)
Update to save settings and allow 25-30 seconds for the AEBS to restart. You can disconnect the Ethernet cable that you used for the setup if you wish
Very important.....now power down the entire network, wait a moment, start the 2Wire first, then the AEBS, then each other device one at a time.
Using your Mac, locate the new wireless network that you named, enter the password and log on to the network.
Your computer will automatically choose the 5 GHz or 2.4 GHz band depending on its capabilities and location in relation to the AEBS.
Strongly recommend that you disable the wireless function of the 2Wire router.