I'd use Java's XML document libraries. It's a bit of a mess, but works.

String xml = "<response>\n" +
             "<returnCode>-2</returnCode>\n" +
             "<error>\n" +
             "<errorCode>100</errorCode>\n" +
             "<errorMessage>ERROR HERE!!!</errorMessage>\n" +
             "</error>\n" +
             "</response>";
Document doc = DocumentBuilderFactory.newInstance()
                                     .newDocumentBuilder()
                                     .parse(new InputSource(new StringReader(xml)));

NodeList errNodes = doc.getElementsByTagName("error");
if (errNodes.getLength() > 0) {
    Element err = (Element)errNodes.item(0);
    System.out.println(err.getElementsByTagName("errorMessage")
                          .item(0)
                          .getTextContent());
} else { 
        // success
}

If you just want to put the content of a String in a file, it doesn't really matter whether it is actually XML or not. You can skip the parsing (which is a relatively expensive operation) and just dump the String to file, like so:

public static void stringToDom(String xmlSource) 
        throws IOException {
    java.io.FileWriter fw = new java.io.FileWriter("my-file.xml");
    fw.write(xmlSource);
    fw.close();
}

If you want to be on the safe side and circumvent encoding issues, as pointed by Joachim, you would need parsing however. Since its good practice to never trust your inputs, this might be the preferable way. It would look like this:

public static void stringToDom(String xmlSource) 
        throws SAXException, ParserConfigurationException, IOException {
    // Parse the given input
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document doc = builder.parse(new InputSource(new StringReader(xmlSource)));

    // Write the parsed document to an xml file
    TransformerFactory transformerFactory = TransformerFactory.newInstance();
    Transformer transformer = transformerFactory.newTransformer();
    DOMSource source = new DOMSource(doc);

    StreamResult result =  new StreamResult(new File("my-file.xml"));
    transformer.transform(source, result);
}

If you can create a string xml you can easily transform it to the xml document object e.g. -

String xmlString = "<?xml version=\"1.0\" encoding=\"utf-8\"?><a><b></b><c></c></a>";  

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();  
DocumentBuilder builder;  
try {  
    builder = factory.newDocumentBuilder();  
    Document document = builder.parse(new InputSource(new StringReader(xmlString)));  
} catch (Exception e) {  
    e.printStackTrace();  
} 

You can use the document object and xml parsing libraries or xpath to get back the ip address.

new StreamSource(new StringReader(str))