I have a mid 2011 macbook air and a new Apple TV but I can't mirror. What's the problem? I have checked display settings and I don't have the "allow mirroring" option there.
What is the problem?
MacBook Air, Mac OS X (10.7.5)
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
Apple TV (2nd and 3rd gen)- How to use AirPlay Mirroring
How to set up and configure AirPort Express for AirPlay and iTunes
iTunes- Troubleshooting AirPlay and AirPlay Mirroring
Apple TV (2nd and 3rd gen)- Understanding AirPlay settings
About AirPlay Mirroring in OS X Mountain Lion
iTunes 10- About playing music with AirPlay
Troubleshooting AirPlay and AirPlay Mirroring
Thanks to the $15 Beamer, AirPlay streaming is still possible on Macs that do not support Airplay and mirroring.
Other solutions are the Air Parrot, StreamToMe, and AirServer.