Your feed couldn't be validated because there was a problem downloading it from the web server. You should try to diagnose this using a web browser, and make sure that the URL you supplied resolves to an accessible file.
One specific case that causes this is a server that claims the file is compressed, but then serves an uncompressed file. You may need to contact your host's administrators to resolve this problem.
Make sure this URL can be downloaded with a browser, then try again.
Not clear? Disagree?
You might be able to find help in one of these fine resources.