My I mac is Mid 2011, but doesn't seem to support AirPlay. Suggestions?

Hi B329,

AirPlay mirroring from a Mac requires Mac OS X 10.8 or later, as you can see from the bottom of this page -

Apple - AirPlay — Play content from iOS devices on Apple TV.

http://www.apple.com/airplay/

AirPlay Mirroring is available with iPad 2 or later; iPad mini; iPhone 4s; iPhone 5; iPod touch (5th generation); and iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer) with Mountain Lion.

You can update your iMac to Mac OS X 10.9 (Mavericks) at no cost using the Mac app store -

OS X Mavericks

https://itunes.apple.com/us/app/os-x-mavericks/id675248567?mt=12&ls=1&v0=www-osx -upgrade-hero

Thanks for using Apple Support Communities.

Best,

Brett L

And you need a AppleTV in your network.