well you usually have to load/enable plugins from the windows>preferences>plugin manager interface
.rar is an archive format.. you need to expand the file first and then read the documentation or readme contained therein as to how it should be installed.
Typically the plugin will be copied to D:\Program Files\Autodesk\MayaX.X\bin\plug-ins
but it may also includes scripts and icons for the scripts or even a shelf which all go into different locations which should be detailed in the plugins install instructions