Press the 3D button on your remote when the video is playing and then choose the top+bottom icon. That should tell your TV to enter 3D mode.
When I watch rented 3D movie from AT&T U-Verse, the movie is split in 2. I.E. the same picture shows at the top half and the bottom half of the screen. Watching the 3D clips supplied by Samsung on the TV, no problems. AT&T support claim wrong settings on TV is the problem. Any help?