In your project, right-click, Add Reference… In the .NET tab, find the “System.Configuration” component name and click OK.
“using System.Configuration” tells the compiler/IntelliSense to search in that namespace for any classes you use. Otherwise you would have to use the full name (System.Configuration.ConfigurationManager) every time. But if you don’t add the reference, that namespace/class will not be found anywhere.
Note that a DLL can have any namespace, so the file System.Configuration.dll could in theory have the namespace “Some.Random.Name“. For clarity/consistency they’re usually the same, but there are exceptions.