The zip archive contains 5 files:

1. ImageDistortion.dlc
2. ImageDistortion.dll
3. OptionBasedLibrary v0.7.9.dlc
4. OptionBasedLibrary v0.7.9.dll
5. readme.txt

Files 1 and 2 must be copied to the paint.net Effects folder.
Files 3 and 4 must be copied to the paint.net folder.

After that start paint.net and you will find the Image Distortion plugin in the Effects menu Distort submenu.