Welcome to the Apple Support Communities
See > http://support.apple.com/kb/HT5404 AirPlay Mirroring requires OS X Mountain Lion and you are running OS X Lion, so you have two possibilities:
1. Open App Store, make a backup, and purchase and upgrade to OS X Mountain Lion, so the AirPlay icon will appear on the menu bar.
2. Install AirParrot to stream the computer onto your Apple TV > http://www.airparrot.com