The dining plan must be added at least 3 days (72 hours) prior to arrival. But one thing to keep in mind is if you have a room only booked, adding tickets and
DDP makes it a package, which are two different reservations. If you do this, your room only will have to be cancelled/refunded and a package booked, requiring a new $200 deposit (or payment in full if you're less than 45 days prior to arrival). Deposits and any money paid toward a room only cannot be transferred to a package, and the same goes for going from a package to a room only.