Depends. Do you already have a package reservation or is it a room only reservation?
Package: If you have a package reservation you can add or modify the dining plan up to 3 days before arrival. Full payment for the dining plan will be required when you add it plus an additional $15 paperwork fee because you'll be within 45 days of arrival.
Room Only: If you have just a room booked you cannot add a dining plan to it. The room only reservation will have to be cancelled and any monies paid towards it will be refunded. You will have to book a new package reservation and pay in full when you make the reservation because you will be within 45 days of arrival.