If your plane leaves at 5:35pm, DME will pick you up at your resort around 2:35pm (three hours before your flight time). They require the three hours because they will probably be stopping at other resorts to pick up guests and they want to make sure you get to MCO in time for your flight. No DME buses leave from DTD. You must catch the bus from your resort at your assigned time. Your DME transportation voucher that's in your DME packet will be stamped with the resort you are leaving from and will be checked by the DME driver.
I posted a story here on the DIS transportation board about an incident that happened when I was at WDW last May. Just as our DME bus pulled up, a rather frantic family came running up, saying that they had gotten distracted at DTD and missed their assigned bus. They were demanding to be allowed on ours. When told there was not room for them and they should have made their assigned bus, the father became irate, but finally admitted they intentionally missed the assigned bus, but thought it was ridiculous to get on the bus three hours before their flight, and assumed they could just catch a later bus. Then time got away from them and they were later than they intended and now was going to miss their flight if they didn't get on this particular bus. When again told that this particular bus already had people assigned to it and it didn't have room for them, the man started insisting that some of us should have to give up our seats and could catch the next bus, so they wouldn't miss their flight. A manager had to be called, but before he got there, the man started saying if there wasn't enough seats on the bus, then they would just stand and then attempted to get on the bus. Of course he was told that wasn't possible either. By then a manager arrived. As we were loading, the family was being escorted into the resort and the man was screaming at the manager that they better not miss their plane. I don't know the outcome, but would guess they wound up having to take a taxi to the airport.