As a PP stated, if there is no ADR availability, then all of the tables have been booked. They don't hold any tables for walk ups. It's just a fact of life that the more popular restaurants book up early.
The only time I would attempt a walk up for a restaurant I really wanted was if I was going at a really slow time of the year. Definitely never when free dining is going on.
If you keep checking daily for Chef Mickeys (try just putting in "breakfast" or "dinner" instead of a specific time), you might still get lucky. Otherwise perhaps you should try for a different restaurant. Mickey appears at other character meals as well.