My desktop is in OS X 10.8.2 (12C60) adn the apple tv works fine with itunes, however the Apple TV icon in the top bar of my desktop has not appeared. How can I activated it?
You're talking about the "Mirroring" icon and not all machines that can run Mountain Lion support it. If it doesn't show up in the Menu bar more than likely your machine doesn't support mirroring your Display to the Apple TV.
Below you may find the guidelines and requirements for Airplay mirroring on OS X:
Hope that helps,
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