We are of the same mind with the dining plan. Though we do use the DVC discount to buy AP's and the TiW card. For us the TiW card is the best purchase we make.
And we often purchase TIW but don't do the dining plan as we don't find value with it. If we did then you just call MS to add it to your reservation. You will pay in full when you add it and it must be done at least 2 days in advance of check-in.