I've had no trouble cancelling ADRs (keep changing my mind). I go to My Disney Vacation, click on dining reservations, sign in, enter my phone number and reservation number (no spaces) and it brings up that ADR. There's then a cancel button to hit. The only tricky thing is you have to have the correct phone number you gave when you made the reservation. I made some of mine over the phone and gave my home number and my online ressies I used my cell. Perhaps you're using the wrong phone number?