Ensure that root element of the XML file will contain something like this:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:PrintData xmlns:ns2="http://iopm.int.kn/iopm4xml/meta/v1.4"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://iopm.int.kn/iopm4xml/meta/v1.4 path/to/xsd/file/schema.xsd">
and then click F8