The nick SEAussie is currently being held by a Services Enforcer. If you are the nicks owner, use /msg [email protected] RELEASE SEAussie password to release the nickname.

what does this mean and where do I use the suggested URL?

Very frustrating.

It means that someone else owns that nickname you are trying to use. If it is you then you need to do what it says and provide the password for the nickname in order to use it.