Help with cheat

I have been trying to use the cheat for the magic mail box but the cheat will not work. I have been typing testing cheats enabled true in many forms but I doesn't work. But my other cheats I type in work on example is the money cheat. But when I try getting the magic mail box does noyallow me. I tried in game mode shift and click and the options don't show up. Does anybody have any suggestions?