There's no Airplay icon on a Mac unless you're running 10.8 and have a supported recent Mac.
On an iPad, open any video in the Videos app and start playing it - you should see the Airplay icon at the bottom right. Or, go to the springboard (double-tap the Home key) and scroll it all the way to the left, where you see the music buttons. The Airplay Mirroring button is there.
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