I might be wrong, but everything I've read makes me believe that the Dining plan cannot be added for ANYONE on the room reservation if the room is paid for with an AP discount. The only way the Dining plan can be utilized is as a packge that includes rack rate room, MYW tickets for each person on the ressie, and dining for each person on the ressie for each night of lodging.
Note: the tix don't have to match the number of nights/days on the room ressie... you could purchase one-day up to ten-day tix with any length resort stay to qualify as a package.
Also, there have been people that got room codes applied to their package- seems that if you get a postcard or email with a PIN, you can apply that rate. But not AP rates.