We arrived at the port around 12 Noon for a 4:30 departure. It was all in the Carnival "Important Docs" booklet the travel agent gave to us. It has all of your coupons for transportation in it.
If you are arriving in Miami the Night before like we did they will most likely have you stay at the Wyndham Miami Airport Hotel like they did most of the other guests we cruised with. Then they sent a bus to pick us up and take us to the Port of Miami and the Paradise. It was very smooth.
Good luck !