This is easy, but requires you to do something completely non-obvious. You can find the Mad Hatter cameo at the very start of the crime scene level in the alley. When you first appear, you're on an upper fire escape. Turn around and look through the window to find Mad Hatter paraphernalia. To unlock the achievement you have to "zoom" view it, which you do by reaching up to your head and holding the grips on the controller to enter zoom mode (this is never explained anywhere...I suppose you're supposed to figure it out by accident).