You need to book your DME reservation with whomever you booked your resort reservation with. They can make the reservation for you. If you booked your resort stay directly through Disney, then call them and give them your DME reservation request. They will need your resort reservation number as well as flight info.
Or, if you used a TA, call them and tell them you want DME added to your reservation.
If you have already made a DME reservation, but needed to give flight info, call whomever you booked resort with and give them the flight info to add to the DME reservation.