Use of unknown namespace:
Even though the feed validator has build in support for quite a large number of popular namespaces, you somehow managed to have found one that the feed validator does not (yet) support.
Check your namespace declaration for misspellings. If you misspell the namespace URI, this information may not be recognized by feed consumers.
If the namespace is spelled correctly, be aware that the feed validator is unable to verify the correct usage of this namespace. Also be aware that this namespace is not likely to be widely supported.
If you know of a namespace that you feel that the feed validator should support, let us know. Please include in your request a pointer to the documentation for the namespace. Such documentation should describe the XML elements and attributes defined for this namespace. More information and the status of current requests can be found on the wiki.
Not clear? Disagree?
You might be able to find help in one of these fine resources.