Penny forgot to add that the "SuperShuttle" is a service at most US airports. They use minivans and charge per person. You can get one outside by the taxi area and no reservations are required. Worst case, you can take a taxi. Ft Lauderdale is not terribly far from Miami, but the shuttle would be less expensive.
But as Penny mentions, contact your hotel and inquire about transportation from Miami airport. They may have service if they know when you are due to arrive.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007