You can try the following procedures,
Configure the LAN adapter of your computer to get IP automatically
Connect your computer to any of the four LAN ports of your Gateway router
Go to command prompt and run ipconfig
Just to determine the IP address of the gateway if you don’t know.
By default gateway routers have 192.168.0.1
you can Verify the IP address of the gateway and write it down.
another option is to logon your web browser http://2Wire.gateway.net
then go to Advance Setting,
Under Private Network,
you can see the IP address of the router.
If dhcp is not enable you can enable and assign, first DHCP address and Last DHCP address e.g. first DHCP address could be 192.168.0.50 and the Last DHCP address could be 192.168.0.99 Meaning the DHCP server is going to lease IPs for 50 DHCP clients beginning with 192.168.0.50 to 192.168.0.99. you can as well make your own specifications here.
Disconnect from your 2wire Gateway router.
Switch on your Bufalo router,
and connect to any of the four Ethernet ports on the Back of the Buffalo router using Ethernet cable. optionally since your two computers have wireless cards, scan for wireless network on range you will be able to see a wireless Network Called BUFFALO connect it.
Buffalo routers have a default IP address of 192.168.11.1
Open a browser and type 192.168.11.1 then press enter
when asked for user name and password, Username: root, Password is admin
in the first main page click on basic setup
Go down and disable DHCP
And modify the Lan IP in the range of 192.168.0.2(depends on the LAN setting of the gateway)
after making this changes click on save and later apply the changes.
Then connect your Ethernet cable to one port on the buffalo router to one of the four ports from the main router(gateway).
This time you will be able to extend your wireless signal.
Remember to go to the wireless settings and change the default Wireless Name BUFFALO and give it encryption key.