Do you have the extreme or express?(you wrote extreme, but described the express)Let's get the stupid questions out of the way...
So you can go online with the wire right? but when you try the wireless it doesn't see the connection.?
The ibook does have the airport card installed?
How many computers are you using? are they wireless?
You've called the ISP, have you called Apple? most ISP techs when it comes to the Mac are clueless.