You should be able to download the dll by simply right clicking on the following link and choose "Save Target As..."
GridMaker.dll
When the "Save As" dialog box comes up, select your Paint.net effects folder and store the dll there.
After it is finished downloading, you should then, right click on the file using explorer and choose "Properties" from the menu. Click the "Unblock" button, then click OK.
Run Paint.net and you should see the gridmaker plugin in the effects menu.