This most likely does not mean that the buses are full. Like they stated, once you are onboard, go to Guest Services and tell them you want to book the transfer from Port Canaveral to MCO. You will usually get a flyer in your stateroom during your cruise about this service.
Once you dock and clear customs, buses are lined up and waiting. Just give the RCI rep your name, who will check you off the list of approved passengers, and tell you which bus to board.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)