Your PHP wasn't complied with the XML libraries. You won't be able use any XML functions until you get your PHP re-compiled with the correct options.
Look at the manual http://www.php.net/manual/en/ref.domxml.php it tells you how to install PHP with DOM support.
If you already have DOM support compiled into your PHP then check that page still, and notice that the xml_tree() function that you are trying to access is actually xmltree().
