Cannot get MacBook Pro to stream to Apple TV

Hi-

I know this has been asked before, but I have yet to find an answer....

I have a wireless home network, a MacBook Pro (mid 2012) running Mac OS X Lion 10.7.5 (11G63b), an iPhone, and iPad, and Apple TV (ATV). I can send video or audio or both to ATV with the iPad and iPhone but not with the MacBook Pro. Going into System Preferences>Displays, the choice to activate AirPlay is missing. No AirPlay icon is in the bar at the top, adjacent to the wireless network icon. I have updated software for all devices including iTunes, which does have the ATV icon.

So, how do I get AirPlay to work so I can stream from MacBook to ATV?

Thanks in advance for your help.