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.
AirPlay Mirroring requires an Early 2011 or newer MacBook Pro and OS X Mountain Lion. You say that you are using OS X Lion so, if you have a supported computer, just make a backup, open App Store and purchase Mountain Lion. After upgrading, you will see the AirPlay icon on the menu bar and on System Preferences > Displays.
If you haven't got a supported computer, download AirParrot to stream the computer to your Apple TV > http://www.airparrot.com