The wiki page is talking about a maneuver node that you would normally use in space to mark a prograde/retrograde/etc. burn. There is no special "landing" node. By placing it on or near the surface, you can gauge roughly how much time you have until landing.
There is currently no stock way to place one exactly on the surface. Just place one as close to the surface as the game will allow.
You don't really need to worry about the inaccuracy of not being able to click on the exact landing point of your trajectory, because unless you're at the very margin of your fuel supply, it doesn't hurt to have the extra few seconds of buffer before the last few meters of your visual approach.
Once you are within the last few meters of your landing, you should be able to judge your height based on your shadow, which should now be visible.
If you are running very low on fuel, you should quicksave and then try suicide burns from different altitudes and not worry about your time to impact as such.