All of the operating systems and their default media players that we have tested, will play the formats that we provide in our latest updates.
MP4 files are supported by Windows and Mac, as well as a myriad of other operating systems (OS).
That being said, your OS may have custom configurations or software on it that have for some reason made your player incompatible with our file formats.
VLC Media Player (VLC) can be installed on about any device, including mobile. Here is a link to their download link: http://www.videolan.org/vlc/ . If that link does not work, simply search "VLC Media Player" in your favorite search engine or app market.