Can you cancel your ressie thru Virgin? Going directly thru WDW or thru AAA will allow you to get the dining plan. Not sure what the deal is, but some agencies don't seem to be able to book trips with the DDP. To get the plan, you must purchase a non-discounted room and at least a one day park pass for each member of your group. It may be that you got a discounted room thru Virgin--thus can't add the plan. AAA seems to be the only agency that gets you a room discount and the dining.
Suggestion--book what you want first, then cancel Virgin. That way, you always have the trip booked!