I believe if you want free dining for this coming fall, you need to be booked by July 30th. To get the regular dining plan, you need to book a moderate or deluxe room, if you go value resort, you get the quick service dining plan. you can obviously pay the difference from either to upgrade to deluxe. If you are trying for this year, i would first immediately check for adrs and booking those before you even book the deluxe dining plan. you dont want to purchase something, and then not get to use all your credits because most places will be filled up by now.
If you are planning for a trip next year, the free dining promotion usually comes out sometime in May, and they do book quickly (the cheaper rooms anyway).