Right-click your project, Add Service Reference... Then under Address put in http://www.weather.gov/forecasts/xml/DWMLgen/wsdl/ndfdXML.wsdl and click Go. Choose a Namespace in the bottom. Then I think you can use it something like:
using AppClass.ServiceReference1;
...
ndfdXMLPortTypeClient service = new ndfdXMLPortTypeClient();
string data = service.NDFDgen(39, -77, productType.timeseries, DateTime.Now, DateTime.Now, new weatherParametersType() { maxt = true });
data
is "DWML encoded NDFD data for a point", whatever that means (though I would guess D* Weather Markup Language). I don't know how to decode DWML, or what that data contains. You may need another method to get radar images.