After a week or so of shouting, screaming and hair pulling I've finally managed to get my prototype app to connect to the new NeSS Data Exchange beta program using C# 2008 Express Edition.
It's been a huge pain in the arse because it uses SOAP and security token extensions that aren't supported natively in the Express editions. To make it work properly I had to download and install Microsoft's Web Services Enhancements 3.0. Once installed I had to use the configuration tool to create a wse3policyCache.config file to use a Username token with my application.
Once configured the following code worked a treat:
//Set up Credentials
const String UserName = "username";
const String PassWord = "password";
//Create new token
UsernameToken tkn = new UsernameToken(UserName, PassWord, PasswordOption.SendPlainText);