This could be something extremely easy for the pro, but it took me hours to set this up. I'm using an old TP-Link wireless adapter. After installing the latest XBian (1.0 alpha 2) on my Raspberry Pi, I found no way to get myself to the terminal, even after pressing "Exit" from XBMC screen.
I had to plug in a network cable and ssh into it. For this version, the login details are:
Whenever you run sudo, the password is "raspberry" too.
Once logged in, exit the blue setup screen. Then run the following command:
You should see a new "wlan0" on top of the existing "eth0" and "lo" entries, if the wireless adapter was detected and installed correctly. I'm not sure what to do if it's not!
Run the following command to install the wireless tool.
sudo apt-get install wireless-tools
Then run the following command to set your WEP settings.
sudo /sbin/iwconfig wlan0 essid <ssid>
sudo /sbin/iwconfig wlan0 key <key>
sudo ifup wlan0
You should get your WEP wifi working by now. Alternatively, you can configure by editing your /etc/network/interfaces to something similar to the following:
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp
iface default inet dhcp
Hope somebody would find this helpful. Do note that WEP is not secure enough for your wifi, always go for WPA2 whenever possible.