Quite a simple process this one:-

• In the Joomla! admin, go to Components - Open SEF - configuration - Google Sitemap. Take note of the XML file name (or rename to something you'll remember). Take note of the sitemap location.
• Open up your FTP program, go to the directory set in Sitemap Location, create a file called {XML file name} (the name you remember earlier) with .xml on the end.
• Chmod (permissions) to either 755 or 777 depending on your server configuration.
• Now go back to Components - Open SEF - Google SiteMap - Generate XML - Save XML file

And voila, one Google sitemap ripe for submitting to Webmaster Tools...