Make sure that the interop dlls you are using correspond to framework version of the add-in. Please read this article: http://help.solidworks.com/2014/English/api/sldworksapiprogguide/Welcome.htm
Also when you say that you are getting the response from SolidWorks are you talking about the runtime response or compile time? Can you add the snapshot of the error?
Regards, Artem Taturevych | Snr. Developer | IC3D ANZ
IC3DSteel – New Steel Solution for SolidWorks
translationXpert – SolidWorks files language translator
Well, just a quick update.
Evidently VS2013 Express doesn't work with SW2013. The clue seems to be the fact that when you install manually the SDK, it creates templates for VS2010, 2008 and 2005. Not for 2013.
On the bright side, Visual Studio Express 2010 works very well with the C# Template provided by SW 2013 and development is rather smooth.
Thank you all for your help and good intentions, especially Artem.