change
<%@ Master Language="C#" AutoEventWireup="true"
CodeBehind="Site.Mobile.master.cs" Inherits="ASP.Site_Mobile" %>
to
<%@ Master Language="C#" AutoEventWireup="true"
CodeFile="Site.Mobile.master.cs" Inherits="ASP.Site_Mobile" %>
(Edit) That is the case if you are working on a website as oppose to a web application.