First off, unlike freebies you'll have to reinstall most payware plugins in order to make them work (a backup of your plugin folder won't do it). Then what is your exact plugin location? If you point Reaper to a common \VSTPlugins folder, it should find all working plugins in that folder and its subfolders when you browse to that folder and hit the "clear cache and rescan directory" button. If it doesn't, the plugin DLL may sit in a sub-sub-subfolder and you probably have to add this path to the existing path.