I think you forgot to attach the file. Anyway, I guess I know the problem. It is ok to import a Wikipedia file (Wikipedia is just one huge Mediawiki installation).
But the newer versions of Mediawiki generated a new XML file that as expected does not validate against the DTD of the old one. The importer works with version 0.3 (http://www.mediawiki.org/xml/export-0.3/) and now Mediawiki is using version 0.4 (http://www.mediawiki.org/xml/export-0.4/).
Next week I will take a look at this problem and if not much have changed from one version to the other I should be able to fix this issue.
If you are a programmer you can take a look at lib/importer/tikiimporter_wiki_mediawiki.php and see if you can fix the problem by yourself.
Thanks for reporting this problem, I was not aware of this new version of the XML file.