URL timeout: DNS lookup timeout

I just tried to add sitetree to my site, and enter in Google Webmaster Tools with the URL of http://sharpenedpenciltutoring.com/sitemap.xml. I got this error message:

We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit. 1 URL timeout: DNS lookup timeout

What do I need to do to fix this?

http://wordpress.org/extend/plugins/sitetree/