The plugin has been installed in the VIERER folder, but there is no LAV in the viewer plugin settings page.
LAV Filters are video codecs that the Video & Audio viewer plugin can use (if DirectShow is turned on in its settings).
They aren’t a viewer plugin themselves, and won’t appear in the list of viewer plugins.
You shouldn't need to use that. Installing LAV Filters is all you usually need to do if you want to use them for decoding.