The most common media format that can be played on a DVD player is : MPEG-1 or MPEG-2. The MP4 files are equivalent to the MPEG-4 file format. This might be the reason why you can't play them on your DVD player. To fix this, try making a Video DVD using InstaBurner. By doing this, your files will be converted to a different format that can be opened with your DVD player.