it happens sometimes. If you close your browser, Disney's site kinda logs you out. To get it to work for me, I:
1)close my browser
2)go to Disney World site
3) click on "My Reservations"
4) it should prompt you to log in
5) when the screen loads, go to the dining tab, and then make dining reservations
6) make your ADR
EDIT: Disney's site seems to be going through upgrades right now, so that may be why. I just logged on and it wouldn't display my reservations, which has happened before when they were doing that.