[Plugin: Multiple Category Selection Widget] How to display message on an empty search results page?

How can I create some content on an empty search results page?

To clarify a little bit – if you go here and do a search in the MCSW for a 2001 Dodge Stratus, which doesn’t exist on the site:

http://www.redkitecreative.com…..nce/about/

The resulting page shows no main content and the right sidebar floated to the left side of the page:

http://www.redkitecreative.com….._type/and/

What page template should I edit to add some content such as ‘your search found no results, please try again.’ It’s not the theme’s search page template.