To add the Dining Plan for guests staying on a DVC reservation you'll just need to call MS and they will add it to the reservation. With DVC reservations the DP must be paid for in full when it is added and it must be added at least 48 hours before arrival. Everyone registered in the villa will need to be on the same DP for the stay.
If this is a reservation for someone other than yourself you will need to make the reservation in their name and then pay for the DP whenever you add it. Magical Express may be added also (no charge for that) once you know the flight info for your guest.
If this is a rental guest, some have had the renter provide a debit or gift card with the appropriate amount available on it to pay for the dining plan - otherwise you'll need to add that expense to the amount you are asking for the rental so you can pay it yourself.