Must have done: You must have befriended the Shaman in Zaire and have Melissa or Leslie enter the Face on Mars and READ STRANGE MARKINGS under the Mayan-looking statue. You will also need the Yellow Crayon from SF and the Lighter from the Airplane.
Once you have Zak land on Mexico, traverse the jungle. It is a randomly generated "maze" with no particular solution; just enter the closest exit and eventually you will reach a pyramid. Just don't re-enter the exits you just came from.
You will reach the pyramid, with 3 possible entrances. You will find yourself in yet another maze; some screens are dark but you can use the WHAT IS verb and USE LIGHTER ON TORCH to light up the room. Once you enter, you are in an exterior "square" corridor of four screens; notice that each one has six doors and green faces and one of them has a pillar on the foreground. Every time you enter a doorway (not tunnel) you proceed to the next screen.
Your task is to enter a tunnel to reach the middle corridor (it has blue faces and each screen has only 4 doors) and eventually the inner one (each screen has 3 doors); the second screen from the one with the pillar has an exit to the inner sanctum with a Mayan statue.
If you don't want to use maps or search, the easy way is this: outside, enter the left entrance and you are at the exterior corridor, the screen with the pillar; enter the door next to you, traverse the screen to the right and enter the door on the wall; you are now at the inner corridor, at the screen with the pillar. Enter the door on the right wall twice, and then enter the doorway to reach the statue.
See here for a map of the maze. See also this schematic highlighting the briefest route.
Once you had Melissa and/or Leslie check the markings on the similar looking statue in the great chamber in the Face on Mars, note down the drawing. Now, as Zak, USE YELLOW CRAYON ON STRANGE MARKINGS and try to draw the same shape. The crystal is released and PICK UP CRYSTAL SHARD. The lights go off so try to find the door, and eventually exit the pyramid by taking the reverse route. Traverse once more the jungle on your way to the airport. Your next stop should be England.