After Lisa goes to the the freak show tent with moe she then goes to library to solve the mystery. You have to start that in order to get Burns to confess. If you have done all that and you still have no option to confess then you are SOL.
I got a push notification telling me to login in to receive Xx free donuts. There was a post early but it got buried from another user. I got 60 I think others have gotten less. Not sure how you get chosen.