[Plugin: Google XML Sitemaps] paths don't match error messge

i’m getting error message from google in webmaster tools. All are the same for all my pages…

We’ve detected that you submitted your Sitemap using a URL path that includes the www prefix (for instance, http://www.example.com/sitemap.xml). However, the URLs listed inside your Sitemap don’t use the www prefix (for instance, http://example.com/myfile.htm)

Do i fix it in webmaster tools, or in wordpress? What section exactly do i have to change to get my sitemap up?

Any help is greatly appreciated. Thank You.

http://wordpress.org/extend/plugins/google-sitemap-generator/