Mountain Lion AirPlay Mirroring (iMac to AppleTV) Not Working

Mountain Lion successfully installed on my iMac.

AppleTV sucessfully networked with my iMac (at least where iTunes and iPhoto are concerned.)

But AirPlay icon is NOT appearing in my menu bar. Any clues as to why this is?

In System Preferences, I've checked "Show mirroring options in the menu bar when available" but still nothing.

Below is a screenshot of my menu bar. Thanks in advance!