First of all, you should not use WEP Wi-Fi encryption! It’s so easily broken into you might as well have an open Wi-Fi network. Also, it’s really a pain to configure on the device so you should use WPA2-Personal, instead.
But if for some reason you need to use WEP, here are instructions for doing so. The page also has a handy calculator for dealing with ASCII keys and even the passphrase mode used on Linksys and some other routers, which I think is kind of cool.