You need to unpack the .zip to get the .dll file, then drop that into paint.net's 'Effects' folder. Mine looks like "D:\Programs\paint.net\Effects". (Re)start paint.net, (if it's open,) and it will show up in the 'effects>stylize' drop down. Fiddle with the settings to your liking, then 'save as' .dds/BC5 (linear/unsigned) compression. Should give you an amber-colored normal map. Let me know if that works.