I'm trying to send data to their API with the correct id and key.
curl -A "Ambient Weather WS-1200-IP" -s -k 'http://rtupdate.wunderground.com/weatherstation/updateweatherstation.php?ID="stationid"&PASSWORD="stationkey"&dateutc=now&action=updateraw'
A couple of thoughts.
(1) Have you made sure that your password is correctly encoded (if it needs to be encoded)?
(2) Are you sure when it says not all fields must be sent that it means that no fields (apart from dateutc, ID and key) need to be sent? That is, have you tried adding an actual reading?
Good Luck