MONITOR OUT FROM FCPX ON IMAC 5k

I have read that Mac pro is able to use the HDMI out for monitoring FCPX by enabling the A/V out. I have an HDMI adapter Thunderbolt to HDMI connected to a LCD TV getting picture as a second display but trying to activate it as a monitor from inside I mac 5k does not work. I cant choose HDMI out in preferences in FCPX and therefore not turn "AV out" to active. Any suggestions ?