Q: Why does my mac not support the apple tv?

You can airplay to the ATV but will not be able to mirror.

In order to mirror, you must have a MID 2011 or newer iMac running 10.8 or newer.

The hardware in older computers do not have what is needed for mirroring.

(You can always look online for software that makes it happen but keep in mind it is not supported by Apple in anyway.)

Specifically the newer Macs have a hardware video encoder, which AirPlay Mirroring needs in order to send a video signal over WiFi without bogging down your computer's CPU. A problem you will face when using third party alternatives.

Now I'd like to point out that AirPlay is not completely unsupported. Your entire iTunes library is AirPlay-able. And there are plug-ins for Safari that convert web videos into an AirPlay friendly format.

If your computer does not support mirroring, look online for Airparrot. It might work.

if you get one on the list yes

http://support.apple.com/kb/ht5404

Are you trying to airplay or mirror?

If airplay what have you tried?

If mirroring, your computer will not work with it. YOU WILL HAVE TO BUY A NEWER ONE- 2011 or newer.

Originally you said you wanted to mirror. Basic airplay is different and can be done on your computer to stream content within iTunes.

If you want to see your computer screen then you will need to use 3rd party software (ie Airparrot) or purchase a new Mac for native mirroring.