When you are at the Plant Electrocution Level 2 slide down toward the holes in the ground and enter the 2nd hole. Make sure to glide into the hole and then glide towards the lighted area. Here you pick up the items. Use the grapple to get out and finish your slide. The key you have just picked up is for the door beside the vines behind the glass. Now Enter and defeat the plant man and you'll get an envelope that has a 'C' on it. When you finish the game this will activate the cheats.