I had uploaded the videos into my Lumia 520 in memory card in MP4 format only, but it was not showing videos in the video player. Using the same technique with songs, they can play. What could be the issue?
This could be happening because you have placed the videos in the wrong folder assuming you are trying to play the videos using Xbox Video. You have to place all your videos in the "Video" folder in your Memory Card. Videos taken through your phone are placed under the "Camera Roll" folder in Pictures, which can be played by opening the Photos App, navigating to the Camera Roll Album and playing them from there.
Update: Microsoft has released an update for Xbox Video which now allows you to play videos under Camera Roll. It now displays the videos under Camera Roll in the Main Page of the app.