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See > http://support.apple.com/kb/HT5404 AirPlay Mirroring requires OS X Mountain Lion and you are using OS X Lion. You have two options:
1. Upgrade to OS X Mountain Lion. Make a backup of your files, open the App Store and purchase Mountain Lion. After upgrading, you will see the AirPlay icon on the menu bar.
2. Keep Lion and use a third-party application like AirParrot > http://www.airparrot.com
Both options are valid, so you can use the one you want
About AirPlay and Airplay Mirroring
AirPlay Mirroring requires a second-generation Apple TV or later, and is supported on the following Mac models: iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer). For non-qualifying Macs you can try using Air Parrot.
Several Apple Articles Regarding AirPlay
Thanks to the $15 Beamer, AirPlay streaming is still possible on Macs that do not support Airplay and mirroring.
Another solution is the Air Parrot.