@rich o brien said:

https://sketchucation.com/plugin-list.php

Or install Sketchucation Tools

https://sketchucation.com/plugin/696-sketchucationtools

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