You will need to call MS at least the day before checkin and add the DP for your renters.
Everyone on the reservation has to have it, so be prepared with names and ages if you do not already have that.
Renters can no longer call MS so all arrangements for DDP and Magical Express has to be added by the member.
Better yet, call at least three or four days prior, since sometimes Member Services is closed the day before, or has technical problems that would preclude making the change to the reservation in time before the deadline.
I agree with Dean. Personally, I would probably handle that transaction with a conference call between you, your renter, and MS...just be be sure the renter fully understands what they are signing up for. I think it's a shaky situation to assume that the renter really understands the terms of the dining plan, so I'd be sure they knew what they were getting.