How to use the XML Sitemap validation tool?
How to use the sitemap validation tool?
How do Sitemap errors affect your site?
Have you validated your XML against the schema given here: http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd
If yes, what was the result?
If not, what is the URL of your sitemap?
If you don't know how to validate the XML against the schema, use http://www.xmlvalidation.com/
Paste the sitemap-XML there, click on "Validate against external XML schema" and paste the schema after clicking the Validate-button.
This will tell you what's wrong with your XML. If you don't know how to interpret the result, please amend your original question accordingly.
Edit: The error was a missing namesapce-URL in the schemaLocation. The first tag has to look like this:
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
Strike the above. Looking at Googles site, their sitemap header seems to be a little longer than yours.
It's on this page: https://www.google.com/webmasters/tools/docs/en/protocol.html
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">