Download the zipfile and extract the folder "Overloading Decal Pack 1" in your Decal Machine folder. Check the General preferences tab of Decal Machine where this folder is located. By default this would be:

C:\Users\<userid>\AppData\Roaming\Blender Foundation\Blender\<Blender version>\scripts\addons\DECALmachine\assets\Decals

Reload the Decal Machine libraries and you're good to go.