Donna in NJ is correct - you had to have a package reservation to start with. If you have a room only reservation, you must cancel it and rebook your reservation as a package including at least a one-day base ticket for each person in your party. Dining plans can't be added to a room-only reservation.
If you are going to get the tickets from Disney anyway, and you don't mind paying for everything at least 45 days before the trip, and you really want the dining plan, switch to the package reservation, assuming they have availability for the resort you want on the dates you want.