This is located in Fisk Tower after you have completed the mission. You need a psychic and gold heat character to get it behind the Kingpin painting. I then went back to the heli-carrier and with the x2 already and just smashed stuff up and unlocked loads of characters. If you keep going up and down in the lift the consoles and boxes on the deck re-appear. It costs 3 million studs, but I just had that after completing the normal levels. If anybody knows where the other multiplier bricks are, please list below. Enjoy!