@rich o brien said:
Or install Sketchucation Tools
ExtensionStore | SketchUcation
3D SketchUp Community for Design and Engineering Professionals.
(sketchucation.com)
Use that to install RoundCorner then in SketchUp under Tools > Fredo Collection > RoundCorner > License click Obtain License
After purchase your license files are available at https://sketchucation.com/webshop/my-licences.php
OMG, thank you so much, it's really confusing how you can purchase the license for the plugin, and you just made it easy lol