This typically means you installed it incorrectly, or installed the 32bit version in to the 64bit plugin folder, or vise versa.
If you downloaded the 32bit version of the plugin, you need to drag the contents of the plugin zip file to:
C:\Program Files (x86)\OBS\plugins
If you downloaded the 64bit version of the plugin, you need to drag the contents of the plugin zip file to:
C:\Program Files\OBS\plugins
Then, load up the version of OS which you downloaded the plugin for, and it should show up.