Wordpress: Useful code to modify the TinyMCE drop-down menu == reference: http://justintadlock.com/archives/2011/05/02/dealing-with-shortcode-madness#comment-335205

custom-drop-down.php

// Your should include this in to your functions theme
add_filter('mce_buttons_2', 'wpse3882_mce_buttons_2');
function wpse3882_mce_buttons_2($buttons)
{
    array_unshift($buttons, 'styleselect');
    return $buttons;
}

add_filter('tiny_mce_before_init', 'wpse3882_tiny_mce_before_init');
function wpse3882_tiny_mce_before_init($settings)
{
    $style_formats = array(
        array('title' => 'Note', 'block' => 'p', 'classes' => 'note'),
    );
    $settings['style_formats'] = json_encode( $style_formats );

    return $settings;
}