It requires Flash Player to be installed.
You can check here: http://www.adobe.com/products/flash/about/ to see which version you should install for your Mac and OS.You should first uninstall any previous version of Flash Player, using the uninstaller from here (make sure you use the correct one!):
http://kb2.adobe.com/cps/909/cpsid_90906.html
and also that you follow the instructions closely, such as closing ALL applications (including Safari) first before installing. You must also carry out a permission repair after installing anything from Adobe.
After installing, reboot your Mac and relaunch Safari, then in Safari Preferences/Security enable ‘Allow Plugins’. If you are running 10.6.8 or later:
When you have installed the latest version of Flash, relaunch Safari and test.
If you're getting a "blocked plug-in" error, then in System Preferences… ▹ Flash Player ▹ Advanced
click Check Now. Quit and relaunch your browser. More advice here:
http://www.macworld.co.uk/how-to/mac-software/unblock-safari-plug-ins-on-mac-360 8065/