I could not get Sketchup 2013 (or v to work with this RoundCorners (RC) plugin on my Win 7 machine.
However v 7 works with the RC plugin
http://support.google.com/sketchup/bin/answer.py?hl=en&answer=60107&topic=13688&ctx=topic

Here's the RC plugin link along with LibFredo6_3.7b... this version works with v 8
http://rhin.crai.archi.fr/rld/plugin_details.php?id=605

To find where Plugins folder is...
past the following into "Window" / "Ruby Console"
Sketchup.find_support_file("Plugins")