How do I use add_action on custom widget?

when calling a function within a class you need to use $this

prefixadd_action('wp_head',array(&$this,'load_face'));

and you will need to populate the value of $sex

public function load_face($instance) {
  $sex = $instance['sex'];
  wp_enqueue_style('style', WP_PLUGIN_URL . '/sample/'.$sex.'.css', null, null, 'screen');
}