How/where did you set this up? WEP will likely be determined by the length of the password you but in on some machines.
The iBook may have an original AP card and not do WPA or WPA2, some would with a firmware update.
Under the Airport icon, click Create new Network, then you'll see 40 bit or 128 bit for WEP key, 40 requires 5 ASCII length, 128 bit requires 13 ASCII length.