DCL has their own Buses that provide transportation to and from MCO to the Port (and the Resorts). There is a charge for the transportation.
If you are planning on using the transportation from MCO to the port, I recommend you actually try to fly in the day before. You can stay at one of the area hotels, then use the free hotel shuttle to get back to the airport in the morning, and then use DCL transportation to get to the port. I recommend this because you don't want weather or mechanical problems to delay your flight and potentially cause you to miss the ship! Also, if you fly in the day before you can get a relaxing nights sleep, and get to the port relaxed and ready to cruise!
On our last cruise we stayed at the Hyatt located inside MCO. It is very convenient, but a bit expensive. You leave your luggage outside your room in the morning and it is picked up and sent to the ship. You meet DCL reps in the lobby and they escort you right to the bus.