How do you unlock professer binns on Lego Harry Potter 5-7?

To unlock Professor Binns in LEGO Harry Potter: Years 5-7, you need to complete the level called "The Order of the Phoenix." After progressing through the story, you will encounter a section where you can unlock him by collecting the necessary character tokens. Once you have the token, simply return to the character selection screen and purchase him to add him to your roster.