Learning is the name of the game with ASP.Net. Today I was trying to use Server.MapPath in my class library. But it always throw an error and Visual Studio wouldn't suggest which Import statement to use.
Finally figured out you have specifically add a reference to System.Web in your project. Apparently "Imports System.Web" at the top of your class file isn't enough.
So, once you add a reference to System.Web, you can now use System.Web.HttpContext.Current.Server.MapPath()