Q: why can i not mirror ?

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

About AirPlay Mirroring in OS X Mountain Lion

iTunes 10- About playing music with AirPlay

Troubleshooting AirPlay and AirPlay Mirroring

Using 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.

Be sure you read the other hardware requirements. It only works on certain 2011 models.

Installed new AppleTV today, 4/5/2013.  connected via CAT 6 cable.   Apple TV sees IMAC shared files and iTunes, however IMAC, Mid 2011, Intel  i7, OS X 10.7.5  DOES NOT  find Apple TV in order to mirror.

Is mirroring hardware dependendent?  Firmware dependent? or OS dependent?

Thanks

Did you bother to read the previously posted links?