I've gone through what I believe to be a correct installation of Simple SEO URL but I'm getting the error info shown below when I click on the Simple SEO URL Manager in Extras. Anybody have any idea where I went wrong?

Warning: require_once(includes/classes/module_installer.php) [function.require-once]: failed to open stream: No such file or directory in /home/mowerpar/public_html/catalog/admin/ssu.php on line 10

Fatal error: require_once() [function.require]: Failed opening required 'includes/classes/module_installer.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/mowerpar/public_html/catalog/admin/ssu.php on line 10