"Your theme needs to be fixed" Error caused Help?

The plugin is causing my wordpress to say there is an error that needs to be fixed below that only happens when Acurax is enabled:

Your theme needs to be fixed for plugins to work. To fix your theme, use the Theme Editor to insert <?php wp_head(); ?> just before the </head> line of your theme’s header.php file.

http://wordpress.org/extend/plugins/acurax-social-media-widget/