LMB? This is not Blender!
Let's say we want to position the catii-iii.xml object at the end of a taxiway. We stop the UFO just above this intersection and press the M key, a Select Model window will open up where the contents of the Flightgear/data/Models/ directory will be shown. Browse this list until you find find your model, then
LMB (Left Mouse Button) click on it to select it, then close the window. The selected model path/name will be display in green text at the bottom left of the FlightGear screen.
Now you may point the mouse on the scenery ground and
LMB again, the CAT II/III ground marking model will be positioned there. Of course, it does not have the correct orientation and it's partly covered by the surrounding terrain, but we can adjust that.