OptionTree installation problem

hi im hosting my wordpress on hostgator vps when i try to install OptionTree plugin i get this error Fatal error: Call to a member function wp_rewrite_rules() on a non-object in /home/thexxxmo/public_html/wp-includes/rewrite.php on line 265 any idea how to fix this

thank you

http://wordpress.org/extend/plugins/option-tree/