Viewing an autostereogram can be difficult at first and some people simply can not see the 3D effect. However, once you can see your first image then other 3D images will be easier to see.
The key is to focus your eyes beyond the screen. One technique is to use a flashlight (or flashlight app) and shine it on the screen displaying the autostereogram. Focus on the light beyond the plane of the display instead of the display itself. Slowly move the light towards the screen and hopefully your eyes will be converge to a plane beyond the screen and see the 3D effect.
Here is an image to try out that was created from the TACK.