Is WIdi compatible with Apple TV

There are restrictions on Airplay in Mountain Lion. It has to do with your video card that comes on the machines that you are using.

Your video card has to be powerful enough to do all the heavy lifting of streaming so that your CPU is unhindered and therefor you can smoothly still use your machine.

This question and answer should help you out and explain things and find an alternative, such as AirParrot.

Here is an Apple Support Article on the subject, listing the restrictions and giving more information on how Airplay Mirroring works.

Here is the excert from the article showing the compatible computers:

iMac (Mid 2011 or newer)
Mac mini (Mid 2011 or newer)
MacBook Air (Mid 2011 or newer)
MacBook Pro (Early 2011 or newer)

As for a Mac Pro, they are not listed as a compatible model that supports Airplay Mirroring, as discussed in this article here.

You can still connect a cable and mirror in that sense, that has not changed in ML. The mirroring feature of using a separate display is a completely different process than Airplay Mirroring.

I hope that this answer helps you out.

Unfortunately, the graphics card in your Mac doesn't support AirPlay as its too old. I have the same problem with my mid-2010 MBP. You can see the list of supported hardware here.

While there is software that will enable this functionality (like AirParrot), I have found it severely under performs on older hardware and isn't really usable. You're welcome to give it a go though. :)