The aprs servers for hams now require passcodes. I don't remember if WeatherLink has a place for you to enter your passcode.
You only need to use the aprs servers if you want your weather data incorporated into the aprs data stream. If you just want your data to go to CWOP, you can just use the CWOP rotation servers, which don't require a passcode. If you do want to use the aprs servers, you'll have to obtain your passcode, and use a weather program that allows you to specify it.
Steve
SoftWx