Did you book a MYW package? Or do you have a room-only reservation and did you purchase your passes separately? The dining plan can only be added to a package. If you booked a room-only ressie, you can't add it. You would have to cancel your room reservation, book a package and purchase at least a one-day ticket for each member of your party.
If you did book a package, then you have to add the dining before arrival. I believe it's at least 2 weeks prior.