Maybe the instructions are unclear? In any case, it worked well for me.
The image looks like it's a 3D shape, say an open ring box with its hinge pointed towards you, with the top half dark and the bottom half light. When you cover the seam with your finger (including the shadows) you see that both halves (except for the shadows, now covered) are exactly the same shade of gray.
The point of the illusion is to show you that perception happens not in the eyes but in the brain. The brain processes the image, and it knows that the bottom half is in shadow, and so when it sees the grey in the bottom it automatically makes us see a lighter color, because it's interpreting what the color would be in full light.