I'll show you how to add the Alpha Mask plugin in this tutorial. This is only how to add "Effects-style" plugins to your program. "File-style" plugins will go into your "File Types".
Also, plugins sometimes have added files that, to me, clog my folder so I make sure to delete all of those before moving my plugin --hence, why I do my drag and drop method instead.
Once you get to the plugins forum, go to the first list of plugins and click "Show":
When you come upon this screen, all of the lists are in alphabetical order and need to be shown in order to see them. Once shown, you'll see the whole list for A-E listed plug-ins. Scan down until you see this:
Click this link to take you to the download page below:
***NOTE: If you are familiar with your unzip program, you can simply unzip it to your "Effects Types" folder under Paint.NET. If not, continue with the tut.****
Once unzipped, you'll need to go to your computer in order to find PDN in your program files, as such:
Open your Paint.Net file and you'll see several folders, but you're only going to need to find the "Effects" folder.
Open your "Effects" folder and then find where you unzipped your file then drag and drop the unzipped plug-in into your "Effects" folder.
Close all windows and open your PDN. You'll go to "Effects" in your toolbar and scan down to "Alpha Mask". Sometimes you have to close down your program as well for the effects to take place. (Most of the time, I don't have to.)
And now you've installed the Alpha Mask plug-in and are ready to use it to create masks out of your chosen papers!