whenever i try this i get this error. i have tried with Cable and wi-fi and always the same.
There are three possible problems:
1. Your router is off/restarting (Which is not likely if you're actually typing this message)
2. You use an airport base station and not an airport express, which would require you to install the utility specifically for that router. (Blame Apple's compatibility problems)
3. You do not use an airport router. (In which case, you port forward the same any operating system does)