After speaking on live chat, they advised me to email reservations@ncl.com and detail the ship, reservation number, departure date, full name, email and address with the subject 'Latitude Merge' and received a reply saying 7-10 day wait for the merge. A day later, I had a query about my booking and spoke to another agent on live chat who managed to sort it there and then for me - she reset my password and told me to log in then asked me to add my cruise onto my account manually.