Are you having difficulty connecting to the Web Service or do you need help with the API ? Can't help with the API as I have never used this service, but if it's a connection issue, post the error message here and I'll try to help.
I don't think its a connection issue. OP is asking how to connect to the service from his webapp.
Like I said, before you can connect to the web service, you should know what methods are available on the API.
Secondly, you need to use some library that enables you to connect to SOAP services. I am not familiar with .net, so I don't know which library supports that, but you could look this up on the web.
Essentially what needs to be done is that you need to write your SOAP call as an object, then using the SOAP library for .net serialize this object as XML and dispatch it to the server. Then read the server response via the SOAP library.
Since I am not familiar with .net, I cannot give you detailed steps, sorry. But if you can share the link for documentation, I could give you further pointers.