unable to air mirror from iMac (yosamite) to apple tv ,no icon visible on mac

I would like to play a home movie on my apple tv but iMovie theatre will only take movies of 11 mins or less.

My movie is 1h 43m,

thought i could air mirror to the apple tv but can't find an icon ,

have already checked wifi is the same on both iMac and apple tv.

any ide