Ultimately the plugins a re supports to put a .dll file in the c:\program files\devnz\gbpvr\plugins directory, and usually a bunch of files in the c:\program files\devnz\gbpvr\skins directory.
If you download one of the plugins and open it with winzip, you'll find it probably just needs to be unzip to you c:\. Next, open in the config app and go to the plugins page. Your plugin will now be listed. Tick the checkbox to enable it (also click the settings button to config the plugin), then start GB-PVR and there should be a new button on the main screen.