Sitemap issue (google thinks its HTML)

I’m not sure what we’re doing wrong, but Yoast puts out one XML page with a list of 5 sitemap.xml pages http://www.organiclifeteas.com/sitemap_index.xml but when I submit that to google it throws up an error.
I tried submitting each page individually and google tries to tell me that the pages seem to be HTML and to try again.
What have we done wrong?

http://wordpress.org/extend/plugins/wordpress-seo/