There is a piano in the Executive Disorder level that, when played properly, will reveal a purple keycard needed to access the secret level. In order to activate it, press the WHITE keys, numbered from left to right, in this order. (Black keys are not counted for numbering purposes) 9, 8, 6, 9, 8, 4. If entered correctly, the floor will sink down, and the purple keycard will be in a little alcove behind you. Once you've beaten the boss, and descend in a long elevator ride, the locked purple door will be nearby, leading to the secret level.