They must have made some other change to that button because hitting the FM and CD buttons as the instructions say, DOES make the display say "DIS ON". It's like it is activated but the button function is somehow disabled or wired differently.
I just got an 08 Mazda3 S Touring and tried this too. I can get it to say "DIS ON" but nothing else works. Does the fact that it says "DIS ON" indicate it's capable but they may have added another step to activate?
This is the *ONE* thing that I really miss from my last car.