I think you are either a little confused, or are confusing me a little. So sorry if this is too simplistic.
What I understand: You enabled WEP on your router, and then connected your iBook back up to it? If this is the case, you DO NOT NEED to do anything further. There is no "enable WEP on iBook." If you are connected to a WEP network, then your connection is secure to the extent permitted by the WEP standard. A WEP-secured router will not let a computer access the network wirelessly without using WEP.
If you have not been able to reconnect, you click on the airport icon in the menu bar, then click on the name of the network, then type the password you created when prompted.
P.S. people here usually prefer that you keep asking related questions like this in your original thread, rather than starting a new one. But welcome to MR and to the world of Macs! 🙂