Add custom CSS classes to the wysiwyg drop-down

function ses_tinymce_css($wp) {
        $wp .= ',' . get_bloginfo('stylesheet_url');
        return $wp;
}
add_filter( 'mce_css', 'ses_tinymce_css' );

or

add_filter('mce_css', 'plugin_mce_css');
function plugin_mce_css() {
  return plugins_url().'/plugin/customTinyMCE.css';
}