If you tried to make a reservation today for 12/6/11, you'll get an error message highlighted in yellow instructing you to log in with a link. That's where you'd be able to put in your reservation number. I was using today as an example, since today is 180 days before 12/5/11.
If you're registered yourself on Disney's sites before, you use that log in. Otherwise you would chose "Create an account". I have not used the site for linking a resort reservation to dining, but that's what it seems to redirect itself.