It's not easy and may take a little while to adjust your eyes.
Tip: Look through the screen (focus your eyes behind it) or try gently crossing your eyes.
Magic Eye pictures, when viewed the right way, give the illusion of a three-dimensional (3D) image. These pictures are called autostereograms because they do not need any special equipment in order to see them (such as 3D glasses or a stereoscope).
Many people find these three-dimensional images difficult to see, and often it is necessary to look at the picture for some time before the image will appear.
If your two eyes have very different diopters, or if one eye is very dominant, then you may have particular trouble seeing the image. In fact some vision therapists use Magic Eye pictures to correct certain visual impairments.
If you are having trouble seeing the shark, then try gently crossing your eyes to different degrees while focussing behind the screen. It is best if your head is about 12-24 inches from the screen, but you can also try moving slightly closer or further away.
Warning: For health and safety reasons, you should not spend too long staring at the screen.
For more information on autostereograms, see this Wikipedia article.