I'm afraid I do not have enough information to point to a problem.
With .Net you usually create a Service Reference, or a Web Reference (the .NET 2.0-3.5 way). Visual Studio may create several files, including a proxy class which you eventually use.
You have to create an instance of the proxy class and then trigger the Login method.
At what point do you get the error?
It's also possible you're organization's security configuration blocks something. Can you reach https://api.clarizen.com/v1.0/clarizen.svc through your web browser?