Apple TV icon not in navigation bar
I checked my mac air by the serial number and its after mid-2011. The software is up to date on my apple tv, but its not showing in navigation bar.
AppleTV 2
To use AirPlay mirroring to mirror the contents of your Mac's display on your Apple TV your Mac must be one of the supported models (which it is), must be running OS X Mountain Lion (10.8 or later) and both devices must be on the same Wi-Fi network.
Make sure that AirPlay is enabled on your Apple TV under Settings and on your Mac (System Preferences > Displays > "Show mirroring options...when available")
For more information about AirPlay Mirroring on your Mac, take a look at http://support.apple.com/kb/ht5404