SmartCrawl: Sitemap Error in Google Wbmaster Tools

Your Sitemap does not appear to be in a supported format. Please ensure it meets our Sitemap guidelines and resubmit.

The above error occurs when I submit to Google Webmaster Tools.

1) Why is that?
2) How can I edit the xml sitemap
3) How can I add custom (non WP) URLs to the sitemap

I am using WP Multisite. Support Access granted