The Mummy player places his smaller magnetic piece to the Treasure Hunters side of the board onto the sarcophagus space (located just to the left of the Pyramid Entrance) near the Treasure Hunter pieces.
The big magnetic Head piece is put onto the Mummy's side of the playing board.
All Treasure Hunters will be able to see the current location of the Mummy player, but the Mummy player doesn't know the position of the Treasure Hunters.