Yes, you select your PAT when you do online check-in. For Silver CC members that is 90 days out.
As to what time...that's up to you. What time to you actually expect to arrive at MCO? Or are you staying overnight at the Hyatt or other hotel? By riding
DCL transfers, you will be allowed to enter the terminal regardless of your PAT, but as PP mentioned your actual boarding number is directly tied to the PAT. But if you are on a flight arriving at MCO at 11am, there's really no reason to take an early PAT because there's virtually no chance you'll arrive that early. If you are staying at the Hyatt or have an early arrival, therefore expecting to get on the first bus from MCO to port, go ahead and grab the earliest PAT you can.
Enjoy your cruise!