See the documentation here .http://docs.oracle.com/cd/E37097_01/doc/doc.42/e35128/sql_utl.htm#BCEHEFHB
To quote from it
Oracle Application Express supports XML documents in <b>Oracle's canonical XML format</b>.
In Oracle's canonical XML format, each element represents a column value, each element is named after the column, all elements that are part of the same row are children of a <b><ROW></b> element, and all <ROW> elements are children of a <b><ROWSET></b> element.
If your XML is not in the above format you cannot use Data Workshop to Load XML data and you have you use other techniques for it. E.g. SQLX functions or DBMS_XML packages.