Some of all us people on a pc might have some problems with the raw video form the Canon EOS 5D Mark II. This i due to quicktime not really supporting hardware on PC’s.
However you can download a small free program called vlc that with a small settings change will playback the video just fine. In general this is a player i will HIGHLY recommend. It plays just about everything and you don’t need to install loads of codec packs and stuff like that. Go to this page for a free download: https://www.videolan.org/vlc/
Out of the box this might not play videos from the 5D Mark II perfectly but a simple change can fix this.
Go to Tool > Preferences in the box that opens make sure you turn on all in the show settings box to the lower left.
Then go to Input/ Codecs > Other Codecs > FFmpeg. Find the setting called Skip the loop filter for H.264 decoding and set the option to all.
*** This trick should work just as well with video from the new Canon EOS 7D
This should fix your problems and VLC should now playback the 5D Mark II video with no problems.