Here's how it works:
On your last night, you'll receive a confirmation and vouchers for your return trip, with a specific time for your bus. It is usually 3 hours prior to your flight time. If your flight leaves at 2:30 PM, then you will be assigned a bus that leaves around 11:30 AM.
If you are flying on one of the airlines that have 'Resort Airline Check-In', or RAC (American, Continental, Delta, jetBlue, United/Ted, Northwest, Alaska and AirTran), you can take your bags to the RAC desk at your resort and check-in for your flight, check your bags and get boarding passes. Then you board your ME bus at the proper time.
If you are NOT flying on one of these airlines, or do not wish to use RAC, you don't need to do anything else. Just present yourself to the ME bus with your luggage at the proper time.
Have fun and enjoy POP!