Did you use a Carnival Vacation Planner to arrange your trip? If so, you can just call them and they will take care of it for you.
1-800-819-3902, Ext. 85648 is the number of the vacation planner that I use. Her name is Rachel Salmeri and I don't think you can reach a planner until after 11am (EST).
I think the website for making payments is up and down a lot.