Whoever you booked your resort reservation with should be able to add DME to your reservation. They 'technically' own your reservation until you arrive at the resort, so they need to make any additions or changes.
Let them know that you need to add DME. If they are lax about getting back to you, I would tell them that you will be cancelling your reservation and redoing it on your own if they can't seem to find the time to make the effort to add DME.
I realize that many TA's, especially those who are Disney knowledgable, can really help guests out. They take the burden of planning off the shoulders of someone who is new to the whole Disney thing. BUT...they can also drop the ball on occasion and make the experience less than wonderful. They really need to do what needs to be done from start to finish.