Here is a link on Google maps: 

https://www.google.com/maps/place/Armonia+Iceland+(V%C3%B6r%C3%B0ufell)/@64.0371517,-20.9577941,17z

Note that you need to look for the road called “Vaðlækjarvegur” which is NOT on Google maps so when you get close use this map and video below. 

The cabin is labeled “Vörðufell”