AirPlay Mirroring requires either a Wi-Fi (802.11 a/g/n) connection or an Ethernet connection to the same local network that both your Apple TV and Mac use. Make sure that your Mac and Apple TV are connected to your WiFi.
If your Mac supports AirPlay Mirroring, an AirPlay Mirroring menu extra appears in the menu bar when an Apple TV is on the same network. Turn on mirroring by selecting an Apple TV from the AirPlay Mirroring menu. The AirPlay Mirroring menu extra icon is blue when AirPlay Mirroring is on.
If you do not see the icon, then you can turn it on via System Preferences > Displays.
If you don't see AirPlay Mirroring in Displays Preferences, then your Mac model does not support it.
If none of the above information helps you resolve your issue, then you may refer to Troubleshooting AirPlay and AirPlay Mirroring article: http://support.apple.com/kb/TS4215
I hope that helps. Good luck!