Getting plugins for Paint.NET is super easy. In the menu bar, navigate to Help | Plugins. Once you select Plugins, a new web page will open in your default web browser. Make sure you are connected to the Internet.

It will take you to http://forums.getpaint.net/index.php?/forum/7-plugins-publishing-only/.

Here, you will see a long list of plugins. To install a plugin, simply copy the extracted files from the plugin ZIP file to the Effects folder in the Paint.NET folder. This is usually C:Program FilesPaint.NET:

If you get stuck, read the post HOW TO INSTALL PLUGINS. It will take you through a step-by-step process. This takes a little time, but it is super easy.