Ok, for starters I know Apple recommends WPA so if that is available you might try that.. have you used this router with other Apple laptops before?
Is there a specific error you are getting? Timed out? Invalid Password? Or just cannot connect?
I'd start troubleshooting by temp. disabling security, see if it can connect then. If you manage that, you know it's a security issue. If you don't, then you'll have to look elsewhere. But I would start there - if it works with WEP disabled, try WPA.