The write-protect screw is on the main logic board, just above the center (with the hinge at the top). It is to the right of the yellow-taped cables, to the left of the heatsink, and has copper-colored tape around its base.

Remove the 3.6mm screw with a Phillips #1 screwdriver.

Using tweezers, gently poke at the copper-colored tape around the base of the write-protect screw. Once enough is peeled up to get a grip, gently pull the sticker up and out.