IIRC you need to be logged into your main email account (via the NTL site), the Add A New Email Address option is in there somewhere.
ie: If my NTL Username was ItsMeHonest
Then my main email account would be the
ItsMeHonest@ntlworld.com one
I would need to log into the site using that user name and the password that I set the account up with and then create the new account from there.
HTH