Mission Mode is not like any game mode; it doesn't have its own button! No, this is just an add-on to other characters' unlock buttons. Instead of having two, it will say one at the top, and that is "Unlock Campaign Start?". If you click on the button, it will guide you through how to get that character using the first option (a special thing to do in a certain game mode). It also can turn on or turn off some buttons so you don't accidentally click them.