In a VS2005 C# project I have added a reference to System.configuration.
In the object browser, I can see the System.Configuration.ConfigurationManager.
In Intellisense Syst
It's just a guess, but maybe you should check if your project is using at least .NET framework 2.0. ConfigurationManager class is availvable since .NET 2.0 as dfescribed here:
link on msdn