You can take DME once in each direction between MCO and your Disney resort. Both trips have to be during the reservation period, so you can certainly fly in the night before the start of your stay and then travel via DME on the day your reservation starts. It’s likely easier to just call DME at 866-599-0951 to book, since the online form requires an inbound flight number/time. Have your date and reservation number ready when you call.