How do I instantiate a JAXBElement<String> object?

When you imported the WSDL, you should have an ObjectFactory class which should have bunch of methods for creating various input parameters.

ObjectFactory factory = new ObjectFactory();
JAXBElement<String> createMessageDescription = factory.createMessageDescription("description");
message.setDescription(createMessageDescription);

ObjectFactory fact = new ObjectFactory();   
JAXBElement<String> str = fact.createCompositeTypeStringValue("vik");    
comp.setStringValue(str);
CompositeType retcomp = service.getDataUsingDataContract(comp);
System.out.println(retcomp.getStringValue().getValue());

Here is how I do it. You will need to get the namespace URL and the element name from your generated code.

new JAXBElement(new QName("http://www.novell.com/role/service","userDN"),
                new String("").getClass(),testDN);