First update your Disneyworld.com online profile to add your voucher number (i.e., your AP status). That should allow you to access the "passholders" link/section.
Once at Passholders section, click on "Passholder Offers" in the middle text below the "Welcome" portion -- and it should take you to the current room discount area. Be careful that you are in the correct offer -- they have a few open right now (like Vero Beach etc).
Then follow the prompts.
As PP said -- if that doesn't work, you have to call.