Blank screen when using Dell Optiplex 9030 AIO as external display (via HDMI input from Macbook Air).

When viewing the Dell Optiplex 9030 specifications, it states that the device can be used as an external display for other devices, via its HDMI input.

I have followed the instructions given in the link below to link-up a MacBook Air as a video input, using HDMI. One user in the thread below says they have successfully done this, although it is not working on my machine. 

http://en.community.dell.com/support-forums/desktop/f/3514/t/19647723

When I switch the display from "PC" to "HDMI" using the OSD (On Screen Display) buttons it only shows a black screen, although my MacBook Air detects that it is connected to the Dell Optiplex 9030, and adjusts its settings accordingly. On the Optiplex 9030, the question-mark below where it says "HDMI" on the OSD disappears, but no signal is displayed on the screen. 

Can anyone please explain a workaround to this problem?