Create App.Config on the Solution

Hey, 

I added an app.config on my solution, because i need to get constants values that Admin user will need to change... ,  i know i can use a list of sahrepoint... i can use web.config... but what i realy  what i want is a app.config file

I created an app.config and i tryed to get the values but i didn't have sucess... 

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="TEST" value="TEST" />
  </appSettings>
</configuration>

public static string _TEST = ConfigurationManager.AppSettings["TEST"];

Thanks for help.