LRC won't allow me to add plug-ins

Unfortunately the mninds at Adobe didn't think to make do not sadd duplkicateds as the default setting when importing images and I messed up.  Now I am trying to eliminate the dups without having to manually select each one.  But LRC Plug in Manager wont allow me to add anything.  I download and see the app on my hard drive but LRC can't find it.  I've tried multiple apps ncluding teekesselchel and Lightroom Duplicate Finder but LRC doesn't seem to be compatible with any of them.  

I'm amazed at the level of complexity ruined by lack of common snese and lack of usder friendliness.  Clearly it is built from a coders only perspective without having engage user groups including novice photographers.  

Any suggestions are much appreciated.