Wrong inclusion of stylesheet

As per the link in original file, I was able to fix the problem. There might be more elegant solution.
I still had problem, due to my theme bluebreeze. The menu was displayed longitudinally instead of horizontal on the top. Editing the themes' style.css appropriately fixed it.

 $css_files = array();
     /* $css_files[] = array(
        'file' => drupal_get_path('module', 'admin_menu') . '/admin_menu.css',
        'type' => 'module',
      ); */

      drupal_set_html_head(theme('stylesheet_import', base_path() . drupal_get_path('module', 'admin_menu') .'/admin_menu.css'));
      if (file_exists(path_to_theme() . '/admin_menu.css')) {
        $css_files[] = array(
          'file' => path_to_theme() . '/admin_menu.css',
          'type' => 'theme',
        );
      }