On Macintosh...

To uninstall your version of Render[in] on Mac OSX, simply delete two files:
  • Renderin_load.rb
  • Renderin.bundle

You will find them in the finder following this path:
"your hardrive"/Library/Application Support/Google SketchUp 8/SketchUp/Plugins/

On MacOS 10.7 (Lion) this folder is hidden, you have to click the Go menu and press the Option key to make the user's Library show up in the menu, and selecting it will open it in the Finder. 

On Windows...

Use the standard Windows Tool to uninstall your version of Render[in]