You can download the original library. Here is the safe, legal way to get it (since Fredo6's official site is the only source):
: You can still download a 30-day free trial with full features. Round Corner Extension Sketchup Free Download
This allows you to offset curved surfaces. If you create a rounded edge by hand once, Joint Push Pull can thicken it, saving you from using Round Corner entirely. You can download the original library