Ensure you've entered a complete WEP or WPA key.
Some brands of wireless routers automatically generate a simple text passphrase from the WEP key. The Kindle is not compatible with passphrases. You must enter the complete WEP key, which contain letters a-f and numbers 0-9. WEP key lengths are as follows:
64-bit WEP keys -- 10 digits
128-bit WEP keys -- 26 digits
256-bit WEP keys -- 58 digits
WPA keys have a minimum of 8 characters and a maximum of 63 characters, and are case sensitive.