You need to have purchased it as part of a package. The time limit for adding to a package or for changing a package may have already passed. If you did not already book a package, you'd have to change your reservation to one.
If you have booked a room only reservation, depending on how you booked it you may be able to cancel it and book a package, in which case you'd need to determine what tickets you need (if you have an annual pass, you can book a room/dining package without tickets).
Room only reservations booked by phone can be cancelled without penalty up to 5 days prior to checkin, but if you booked by Internet you incur penalties if you cancel within 45 days of checkin.