Assuming you use the latest version of SS:

Download the latest build here:
http://doc.silverstripe.com/doku.php?id=modules:googlesitemaps

(FTP to your ss site): Overwrite all files in the googlesitemaps directory with the new ones you just download
then browse to: http://yourwebsite/db/build

then login to http://yourwebsite/admin

the UI of google site maps ranking is there. Now you have the following problem: "sitemap.xml is not being generated"

I don't know how to solve this, but maybe someone else can. Please let me know.

Thank you for the quick reply:

Here's how I changed the _config.php located in the mysite folder:

<?php

global $project;
$project = 'mysite';

global $databaseConfig;
$databaseConfig = array(
   "type" => "MySQLDatabase",
   "server" => "localhost",
   "username" => "***",
   "password" => "**",
   "database" => "***",
);

// Sites running on the following servers will be
// run in development mode. See
// http://doc.silverstripe.com/doku.php?id=devmode
// for a description of what dev mode does.
Director::set_dev_servers(array(
   'localhost',
   '127.0.0.1',
));

// This line set's the current theme. More themes can be
// downloaded from http://www.silverstripe.com/themes/
SSViewer::set_theme('highway');

//enabling 3rd party software
GoogleSitemap::enable();

?>

Still no sitemap.xml . I have CHMOD the root folder to 777, flushed the site and logged in and clicked save on a page. Still no effect. Any ideas?