The image above is that of your default Left Side editor. It works basically the same way as the Main Page Editor. The only difference is the widget, which is the small image that you can add right before the links on the left side.
Tips to adding your own Left Side:
1. Erase all of the HTML coding in that box and replace it with your own coding. Don't forget to remove the Powered by and the Case's Logo in your coding, as this automatically adds those to the page.
2. Remove only the { MENU } and insert your coding.
And that's it! No need to use the onLoad command anymore.