I browsed the stack trace that you have posted earlier. Because of that I thought that you were already using apache axis2 to call the web service.
Axis2 implements many of the WS-* specs. Find out for yourself if axis2 is right for you.
Axis2 uses the StAX API for XML processing. OMElement is the StAX equivalent for a DOM Element.
The payload is the XML portion that you put into the SOAP body of the SOAP message. It contains the method to call as well as the parameters.