The visual studio shows the System.Configurations namespace by default. But it doesn’t show that namespace content (such as System.Configuration.Configuration class) by default.
You have to add a new reference to System.Configurations namespace explicitly.
This is more confusing as it allows you to add using System.Configuration; in the class but then doesn’t show the contents of it. It doesn’t give any information that can make you think that you need to add reference to the same namespace.
So if you cannot find the contents of this namespace, then simply add new reference to System.Configuration which can be found under .Net section in the Add reference window.