Infinite SEO Sitmap not accepted by Google Webmaster Tools

I set up the Infinite SEO plugin for the purpose of using the sitemap functionality. I originally used the simple sitemap plugin, but I kept getting an error when submitting the sitemap to google webmaster tools, the error was:
"Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead."

So I tried Infitite SEO, and I am still getting the same error. The sitemaps for my sub-sites seem to be accepted OK, but the sitemap for the main website is not, it consistantly says:
"Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead."

What can I do?
Thanks,
-Chase