you may add the dining plan on
DVC stays, but there are no discounts for the
DDP (and certainly no "free dining.") if you are renting, you will need to have the DVC member add the dining plan at least 48 hours prior to arrival, and you can pay for it at check-in.
if you buy an AP - DVC members get a discount on APs, renters do not - you can get a
DDE card.
if you are a DVC member, then showing your DVC card will also get you
10-20% discounts at a variety of on-property restaurants. these discounts do not apply to renters.