First download the plugin : Media Center PluginAfter you download the plugin open up Safari preferences and go to extensions. Pull up the Media Center Plugin and you will see these options:
The options you click above are what will show up when you right click on a media file. Example: Use a youtube video.
As the video is playing, just right click and you will presented with the above options. You can download the video, open it in Quicktime or play it via AirPlay to AppleTV.