Thanks for a great effect!
The download worked fine for me, as did the plugin.
1) Download the DLL fille.
2) Copy/Paste the DLL into your Effects folder.
3) Now when you open Paint.NET and you click your Effects tab from the menu, you will notice this effect added to the bottom of your list of effects.
4) Load an image that is used for your models texture and grey scale it (turn it black & white).
5) Now use the effect on that greyscale image and you now have your normal map.
6) save the normal map and use it with whatever program your model will be used in.
Here is a quo