One can only get the Dining Plan addon if you book a WDW package through WDTC. All of the internet hotel/travel sites set up rates for room only reservations, and they
cannot be converted to packages.
A package consists of a room reservation, and MYW tickets (unless you are an AP holder or DVC owner- Disney has special "packages" for them handled through their own system/reservation desk. I am talking about the general public). If you have a WDTC package, then you can add a dining plan, or one of the deluxe add-ons that cover food and entertainment/tours. Dining plans have to be added and paid for I believe 72 hrs before check-in. I believe DVC owners/guests renting points can add and pay for the dining plan at check-in.
If you book through either
AAA or another
travel agency, they are buying these same packages through the
travel agent side of WDTC, and are being paid a commission by Disney.
You cannot buy a room through expedia, and then tickets from Undercovertourist and then call disney asking to change these to a package so that you can get the dining plan.