Mirroring isn't working...

I have a 2nd Gen. AppleTV and a Mac Pro with OS 10.8.2. The airplay works through iTunes but i'm still not able to get the icon in the tool bar for mirroring (i've also gone through the system preferences and the option that shows up on the tutorials to mirroring isn't there either). This is the only reason i bought the Apple TV and it doesn't work.

I know it's not becase anything is old because the computer and apple tv were bought mid 2012. Also, i'm using a Time Capsule as the router (which was purcahsed at the same time).

Any thoughts to why it isn't working?