Just bought a BENQ GW2765 monitor based on friends' recommendations & online reviews. Came to this support page because I was also baffled about how to connect it to a MacBook Air and get it working. The monitor came with some cables that were of limited use and the CD manual which gives detailed instructions on how to set it up on Windows 7, 8 & Vista.
Bought a Thunderbolt to HDMI input cable - LOGiiX brand CAD $40.
Connected everything, turned on power for the monitor, re-started MacBook Air, etc, etc. Nothing displaying. Monitor gives one display clue by telling you there is no input cable connected. So to change the input mode of the monitor you have to play with the buttons located (on my model) on the right side of the screen. One of the buttons takes you to the menu and that's where you can select the input mode. Once I selected HDMI input voila the display lit up and is working perfectly.
So SarnieV you have to determine which of the unlabelled buttons on the monitor is the menu selection which will take you to determining input mode.
Good luck!