This seems to be a common issue that students and teachers are experiencing lately.

When double-clicking on a .m4v movie file, it tries to open it using Mocha AE CS6, which results in an error and the movie never opens. 

This error occurs because your movie should be opened using Quicktime Player, not Mocha AE CS6. For some reason the computer you're using gets set to open the movie files using Mocha AE CS6, instead of Quicktime.

The Fix

1. To fix this, control-click (or right-click) on your movie file.
2. Select Open With -> Quicktime Player

To Fix Permanently

1. Control-click (or right-click) on your movie file.
2. Select Get Info
3. Under Open With, click on the dropdown menu and select Quicktime Player.
4. Finally, click Change All...