<appSettings>
    <add key="webpages:Version" value="3.0.0.0" />
    <add key="webpages:Enabled" value="false" />
    <add key="ClientValidationEnabled" value="true" />
    <add key="UnobtrusiveJavaScriptEnabled" value="true" />
  </appSettings>

<appSettings>
    <add key="Greeting" value="Hello World" />
</appSettings>

using System.Configuration;

public ActionResult GetAppSettingsProperty()
{
     var message = ConfigurationManager.AppSettings["Greeting"].ToString();

     return Content(message);
}