If plug-in you've just installed doesn't show up on plug-in list in Reaper, please walk through following steps:
First of all make sure all Software Requirements are met by your setup.
Additionally if you're Windows user:
Try out the solution described in this entry at our FAQ. It's possible that some system runtime libraries required by plug-in are missing. This concerns especially relatively fresh Windows installations.
Still no success? Try doing full plug-in rescan in Reaper: