After bashing my head against this issue for days with a new MacMini and Apple Retina display, trying various combinations of the Terminal command (sudo killall VDCAssistant) and resetting SMC, I stumbled over the El Capitan 10.11.5 Combo Update installer and decided to try it, even though I was already at 10.11.5.
Surprisingly, it allowed me to install, but there was no immediate success. As it always had, the sudo killall VDCAssistant command still came back with "Process not found"
Today, sudo killall VDCAssistant worked - and the computer now sees the camera!
Download OS X El Capitan 10.11.5 Combo Update