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AirPlay Mirroring is only available in OS X Mountain Lion > http://support.apple.com/kb/HT5404. First, open > About this Mac and check Version.
If it's 10.7, you have two possibilities:
1. Open App Store and purchase Mountain Lion. After upgrading, you will be able to use AirPlay Mirroring on System Preferences > Displays, or on the menu bar. Note that, if you bought your Mac after July 26th, 2012, you can upgrade to Mountain Lion for free > http://www.apple.com/osx/uptodate/
2. Use AirParrot > http://www.airparrot.com
If it's 10.8, open System Preferences > Displays, and select your Apple TV in "AirPlay Mirroring"