Hi Srinivas,
The key is the session id.
The key is assigned to you on successful log-in.
It will be used for your DI Server request during your current session, and will valid until either you log out or time out.
For the error 404:
1. Keep your DISSample project running.
2. In your WSClient project, goto your references, there will be a web reference to 'sample'. Remove and re-add this web reference.
3. Try and run again.
Regards
Edy