Immediately after you had set up your DEST & imported the XML Tester to your workspace ,you should have ran an XML transaction.It may resulted in the following error.
Exception in initialize javax.naming.NamingException: Failed to initialize the ORB [Root exception is org.omg.CORBA.INITIALIZE: Unable to init plugins vmcid: IBM minor code: 504 completed: No]
java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 0
Error in testRequestMethod :java.lang.NullPointerException
Add the jars com.ibm.ws.admin.client_6.1.0.jar and com.ibm.ws.webservices.thinclient_6.1.0.jar to the build path of the XML tester.