Here is a handy little guide I made to help you with installing plug ins for Rhino.
Click on “File” and go down to properties.
Click on on “Plug-ins” on the left hand column (1), click “Install” and select your plug-in file (2), e.g., “EdgeSoftening.rhp”. Click “OK” (3).
That’s all there is to it! Some plug-ins come with their own installer. Those are handy, but the regular ones that don’t are super easy to install.
Hope this helps!