You probably can't, but it doesn't matter.
You can still use the dining plan to pay for meals, even if there is no link.
Except for CRT and shows, the ONLY advantage of linking the ADRs to your reservation is the consierge can give you a print out of the ADRs linked to your resort reservation.
For CRT and the shows, you need to link it to your dining plan to avoid paying for it at the time of booking (but if you do, you can still use the dining plan when you show up and get a refund on your credit card).