This does not work for me, my macs are brand new. so annoyed, I would love this feature, if it worked.
I've tried every step, in the instructions. so disappointed.
MacBook Pro (Retina, 15-inch, Mid 2015), iOS 9.1, iMac
Are you certain that the iMac is one that supports Target Display Mode?
https://support.apple.com/en-us/HT204592
Are you certain that you are using the appropriate connecting cable?
Ciao.