Be sure the content is 3D content.
Hold the top (up) button on the left side ear can for 2 seconds. If enabled properly you should now see the change from two side by side images to one 3D image.
Please Note:3D videos must be side-by-side in 720p format. You can test videos by searching “3D SBS 720” or 3D side by side 720” on YouTube.