I'd imagine it's just a matter of using the tool(s) that came with your distro to setup WEP for the connection. The first place to start looking is your distro's forums/wiki for WEP setup (and you might want to consider something a little more robust than WEP, I've managed to ... ahem... "test the vulnerability" of a WEP wireless AP and been on the AP in less than an hour (most of which was an attack to generate packets and collecting those packets).