ConfigurationManager Class not exist on .NET 4.5 Framework

I just start working with .NET Framework 4.5 of C#. Am using Windows Form Application. I have do the needed imports such as :

using System.Configuration;

But actually the ConfigurationManager class seems that it is not exits. I have tried to use ConfigurationSettings instead , but Visual Stdio telling me that it is obsolete and replaced by ConfigurationManager !

This is chunk of code related to the problem :

Int32.Parse StartingMonth = int.parse(ConfigurationManager.AppSettings["StartingMonthColumn"]);

So , could you please help me ?