I do have an iMac 21.5 inch Mid 2011 as it explained on Apple Support page but there is no "Airplay Mirroring" option in the "Display" menu.
If your profile is right mAli34 it is your OS. To mirror you need OS X 10.8 Mountain Lion which you need to purchase from the App store, mirror is a feature new in that OS. You Mac will support it as it is mid 2011.
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