WordPress.org

1. Upload the whole plugin folder to your /wp-content/plugins/ folder.
2. Go to the Plugins page and activate the plugin.
3. Use the Options page to change your options
4. Use the following code in your theme template most probably Search Results (search.php)

Also, CategorySearch is fully compatible with wp_list_categories()

This mean you can further tweak the category search results by adding additional parameters. Example:

$CategorySearch->Show('orderby=count');