Is there any chance you can post your modified code on the developer forums? I did not put in the full XML string into the sample files, mostly because there are dozens of ways to put together the XML for the various HTTP requests. There are two main reasons why you would receive a 415 error, see our Common Errors thread here.
Once you post your code, it will be easier to tell which reason you're receiving the error.
Dave Berard Senior Product Manager, Constant Contact