Disney does not allow "stacking" of discounts.
Free dining will cancel out any other discount you are getting.
This particular discount is available only on a room/tickets package of no fewer than 3 nights and no more than 14 nights. The package must include a full price room and at least 2 day park tickets with the addition of park hopper, Water Parks and More, or both. Reservations that fit these parameters and have a check-in date within the promotion period are eligible for the promotion, based on availability.
If you already have a resort booking, you must apply the discount to your package (which will cancel out any other discount you booked with), or you must change a room-only booking to a resort package with tickets. It can't be just added automatically to an existing reservation. If you don't have all the necessary components, you'll have to add them. If your resort and/or room category is not available with the promotion for your dates, you'll have to change the resort, the room category, or both to get the promotion.
If you booked a room-only reservation with the intention of purchasing discounted tickets from an authorized reseller, you'd have to still purchase at least 2 day tickets as part of a resort package in order to get free dining (even if you have already purchased the tickets from the reseller).
