If your online reservation is a package (i.e. you included ticket purchase during the reservation process) you should have been able to add the DDP at the same time.
If your online reservation is room-only, you will need to cancel that and reserve a package.
If your reservation is already a package but you missed the DDP option, you should call CRO.
It's $38.99 per adult per night of your package, and $10.99 per child per night.