>>>Mac OSX
Which version?
The current version of Adobe Flash Player (which Youtube uses to display videos) is only designed for use with Mac OS X 10.6 to 10.9. You probably need to update your Flash Player from here:
http://get.adobe.com/flashplayer/otherversions/
With Windows, I'd strongly recommend uninstalling 'Adobe Flash Player Plug-in' first, then restarting your computer before using that download link. If you can work out how to uninstall the plug-in on a Mac (which I know almost nothing about), I suggest following the same route.