MyRoundCube is a collection of plugins you don't need to edit any of the base files. All you need to do to use a plugin is put its folder in the [RC root]/plugins folder then add the plugin name (same as the plugin folder name) to the plugins array in the [RC root]/config/main.inc.php like:
$rcmail_config['plugins'] = array('plugin1', 'pugin2', 'etc');