There's a blurb about the DDP on the dvcmember.com site. And this is probably the most important part:
Remember that Disney Vacation Club Members may book the Disney Dining Plan only if they do so at least 48 hours before arriving at their resort. Call Member Services for more details.
Just like the DDP was booked in advance for typical resort stays, it also has to be booked in advance for DVC, as well.