The Plugins folder in SketchUp 8 will be under Program Files/Google/Google SketchUp 8/. You must make sure you have full permissions established for the Plugins folder before you can install plugins there. If the plugin was downloaded as a ZIP file, you must extract the contents of the file to the Plugins folder making sure you respect the file structure inside the ZIP file. If you don't the plugin won't work. Specifically, which plugin are you referring to and where did you get it?