Hello,
if the static XML file placed at the webroot is not showing probably the path is wrong. Normally it should be the domain link + the name of the sitemap file:
http://domainname/sitemapfile.xml
If the path is correct there could be some directives in the htaccess or server that avoid opening XML files.
However normally nobody need anymore a static XML sitemap at the webroot, it's an old solution that requires manual update.
Use instead the dynamic link for the XML sitemap generated in realtime and add this to the robots.txt that will have the same effect for search engines:
http://domainname/index.php?option=com_jmap&view=sitemap&format=xml
More info at this link:
http://storejextensions.org/faq/when-i-add-new-contents-is-my-sitemap-updated-automatically.html
Cheers,
John