devloper -2 Newbie Poster

in my project - there are 3 architecture
(1) Presentation(WPF)
(2) WCF
(3) DataAccess(ClassLibrary)
but problrem is that in DataAccess (Class Library) through i can insert,update and delete data..when i put static path(of xml file) every thing will ok but when i add key and value in web.config and write this
System.Web.Hosting.HostingEnvironment.MapPath("~/folder/file"); in access file

it shows assembly refrence(Service Refrence) not found in WPF...but i already add service refrence in WPF..
When i write Server.Mappath At that same things occur..Is there any communication necessary between WPF and WCF(Service) to access d path..
I m doing this thing for last 3 days..but not get proper solution..
Thanks