Problem with sitemaps and seo wordpress mu style

Installed the plugins as per instructions but I am getting an error when trying to activate the sitemap plugin.

The error reads as: Fatal error: Call to a member function GenerateSitemap() on a non-object in /home/sites/wpcornerads.com/public_html/wp-content/plugins/sitemap.php on line 41

Will appreaciate some help on this, thanks.

Zulf