Installation: 

copy & paste the files from the main folder into Decal Machine addon folder, which is located on your system drive under assets -> Decals (Blender addon folder is located on your system drive, under users/“yourName“/AppData/Roaming/Blender Foundation/ Blender/“Blenderversion“/ Decal Machine/assets/Decals).

If you use a custom Asset/Decal location use that one.