In visual studio, use the "Add Web Reference" feature and then enter in the URL of your web service.
By adding a reference to the DLL, you not referencing it as a web service, but simply as an assembly.
When you add a web reference it create a proxy class in your project that has the same or similar methods/arguments as your web service. That proxy class communicates with your web service via SOAP but hides all of the communications protocol stuff so you don't have to worry about it.