I don't think you're taking the SOAP envelope into account... Your generated JAXB Unmarshaller won't know anything about the Body or Envelope tags, it will be expecting your createSession to be the root element hence the "unexpected element" error.
You need to extract the content from the Envelope first, you can do this with message.getSOAPBody().extractContentAsDocument() if you create a SOAPMessage object from your content first.
It's quite fiddly to do, here's a working example from my blog
String example =
"<soapenv:Envelope xmlns:soapenv=\"http://schemas.xmlsoap.org/soap/envelope/\"><soapenv:Header /><soapenv:Body><ns2:farm xmlns:ns2=\"http://adamish.com/example/farm\"><horse height=\"123\" name=\"glue factory\"/></ns2:farm></soapenv:Body></soapenv:Envelope>";
SOAPMessage message = MessageFactory.newInstance().createMessage(null,
new ByteArrayInputStream(example.getBytes()));
Unmarshaller unmarshaller = JAXBContext.newInstance(Farm.class).createUnmarshaller();
Farm farm = (Farm)unmarshaller.unmarshal(message.getSOAPBody().extractContentAsDocument());
It seems that if you don't declare your namespace in your schema .xsd file then you'll see the error you have.
I created a dummy schema with a root element createSession and by adding the targetNamespace attribute and regenerating the JAXB classes the error disappeared
<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://example.com/v2"> <!-- targetNamespace essential for JAXB to work-->
<xs:element name="createSession">
<xs:complexType>
<xs:attribute name="foo" type="xs:string" use="required" />
</xs:complexType>
</xs:element>
</xs:schema>
