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See > http://support.apple.com/kb/HT5404 AirPlay Mirroring is only compatible with Mid 2011 or newer iMacs, and OS X Mountain Lion.
If you have a Mid 2011 iMac or newer, you can upgrade to OS X Mountain Lion in order to get AirPlay Mirroring. To upgrade, make a backup of your data, open the App Store and purchase OS X Mountain Lion. When the download finishes, the OS X Mountain Lion installer will appear, so follow the steps to upgrade.
If your iMac isn't supported or you don't want to upgrade to OS X Mountain Lion, you can use a third-party app as AirParrot > http://www.airparrot.com