The MYW Dining plan has to be purchased as part of a room and ticket package. AP holders, since they do not need the ticket portion, do not qualify for the Dining plan (you can't apply AP room discounts, and you'd have to purchase at least a one day MYW ticket for each member of your reservation to get the Dining plan- not worth the extra $$$).
AP holders can, however, get the Disney Dining Experience card, for $50/year. The DDE card entitles you to a savings of 20% off the total bill for up to ten in your party at almost every Disney property sit down restaurant. There are some other discounts, as well.
If you eat sit down meals at least a couple of times on your vacation, it might be worth it to check out the DDE card.
