The WEP key it wants is the first hex key listed in the Linksys access point. To enter it correctly, you must either select the "hex" input option button, or start the entry with a '$' (without the quotes). The '$' tells the AirPort card to read the input as hex, otherwise it will read it as simple text, and the key will be wrong.
The next thing you need to do is get a new access point or router that supports WPA. WEP is horrendously insecure, whereas WPA is extremely secure. Further, the WPA standard includes a standard way of interpreting keys/passphrases, so no matter whose products you use, they will all accept the same key in the same way.
Finally, remember to ask networking questions (this is not really about your iBook) in the Networking forum, where a lot more network-savy people will look for questions and post answers.