That's fairly normal - they use a pre-configured MadVR/LAV set-up, which by and large is not expected to be updated by the user, as newer builds will eventually contain updated versions, meaning anything you have installed will continually be overwritten. That said, they are contained here - C:\Users\AppData\Roaming\JRiver\Media Center 19\Plugins. AppData is a hidden file, so you may have to unhide it.