System.Configuration.ConfigurationManager Questions

I know that you can use System.Configuration.ConfigurationManager in a web application that has a web.config file in the web application.  But, how do you add the System.Configuration.ConfigurationManager class into a Class Library application?  I found an application where another one of my coworkers was able to add the ConfigurationManager to the Class Library application but  I am unable to.  Anybody have a clue how to add this to a C# 4.5 Class Library Application?