If you've created your website in Google Sites, under Google Apps, you can submit the following sitemap which will work perfectly fine in Google Webmaster Tools:

http://www.domain.com/system/feeds/sitemap

Just replace the portion of "http://www.domain.com" with your primary domain.

If you've created and verified a site using Google Sites, we'll automatically generate a sitemap. You can then take this sitemap and submit it to Google .

If your site is hosted at Google Sites  (for example, http://sites.google.com/yoursite), your sitemap URL is:

http://sites.google.com/site/(your_site_name)/system/feeds/sitemap

If you created your site using Google Apps , your Sitemap URL is:

http://sites.google.com/(your_domain)/(your_site_name)/system/feeds/sitemap