You can unlock Bonnie by Buying the Screwdriver inside of Marie's Shop. Once you get the Screwdriver it will be available in the Break room. Grab the Screwdriver and open the vent that's in the security room. In the Vent, Go right and keep straight, then you'll be inside the Arcade room. keep in mind, once you unlock Bonfie, she will be permanently unlocked. Hopefully this helps you out. (P.S put the coins you see spread around the Night club in the machine when you first walk in to keep her occupied in arcade)