Hi, there!
My SketchUp version is 19.2.221. I have managed to successfully download the LibFredo6_v9.rbz Roundcorner Extension. I think that I have enabled it: you can see my Extensions Manager in the image below. My problem is that I don’t know how to actually pull up the RounCorner tool in the toolbox: I cant seem to find it. I’m using a Mac. Thanks!!