Submit a sitemap to Google Webmaster Tools

Go to Site Configuration -> Sitemaps. Click on Submit a Sitemap and below on the pop up text field.

    system/feeds/sitemap

Your sitemap URL will be:

    http://site.google.com/a/(your_domain)/(site_name)/system/feeds/sitemap

Or

    http://(site_name)/system/feeds/sitemap

See Google support link for more information.