The way I understand it, the offer is for folks that are staying at a WDW resort until Dec. 31- while they are there, if they book another trip for some time from mid August through September (has to be a room+ tix+ dining pkg), they will recieve a $$ discount equaling the cost of the Dining plan.
If you didn't book while you were still onsite, call CRO ASAP and see if they will extend the offer to you. You might be allowed to book.