wpmu-dev-seo Fatal Error.

Hi there,
I have this error "Fatal error: Class 'WDS_Core_Admin_Tab' not found in /home/wpggroup/public_html/home/wp-content/plugins/wpmu-dev-seo/wds-files/wds-sitemaps/wds-sitemaps-settings.php on line 233"

My website is shop.capillion.com. If I click on the shopping cart icon from the mainpage (under Recent Products), It will bring me up the lightbox and when i add to cart, it will have the fatal error. If I click add to cart from any other page or method it works.

I tried replace the plugin file - "eshop" to restart. But it does not work. What else can i do?

Please assist.