To clarify...
The dine plan is NOT sold separately but must be purchased as part of a Walt Disney Travel Company Vacation Package or Disney Vacation Club Vacation Package.
If you are a DVC member, you would book a DVC Points Dine Package through member services. This package includes a stay at a DVC Owned Resort (booked using points) plus the Disney Dining Package for everyone in the room for the entire length of stay. You must book or upgrade to this package at least 3 days prior to arrival.
If you are an Annual Passholder you would book a Passholder Dine Package directly with Disney or through your Travel Agent. This package includes a stay at a Disney Owned Resort (with or without a passholder discount) plus the Disney Dining Package for everyone in the room for the entire length of stay. You must book or upgrade to this package at least 3 days prior to arrival.
If you are neither, you would need to book a Magic Your Way Plus Dine Package, again directly with Disney or through your Travel Agent. This package includes a stay at a Disney Owned Resort, a Magic Your Way Ticket for everyone in the room, plus the Disney Dining Package for everyone in the room for the entire length of stay. You must book or upgrade to this package at least 3 days prior to arrival.
If you already booked a Room-Only Reservation, you would need to be rebooked as one of the above packages. The room category you have booked can be transferred to the package, however any discount you may have used on the room-only may or may not be available with a package. Also any deposits or payments you have made with a credit card will NOT be able to be transferred. You would need to post a new deposit (or make full payment if within 45 days of arrival) for your package and the funds from your Room-Only Reservation will be refunded back to your original credit card.
If you already have tickets, but want to book the Magic Your Way Plus Dine Package, you would need to re-purchase at least a 1 Day Magic Your Way Base Ticket for everyone in the room as part of this package. If you existing tickets are unused, you may be able to contact WDW Ticket Services to be refunded for your separately purchased tickets by providing your new Package reservation number and returning the unused tickets.