winform .cs 类文件中读取App.config数据连接字符串

我们把数据连接字符串放到配置文件中,但如何读取了,以前在VS2003中使用
    ConfigurationSettings.AppSettings["connectionstring"].ToString();
    来读取配置的连接,但在VS2005中却被编译器提示说:
    警告:“System.Configuration.ConfigurationSettings.AppSettings”已过时:
   “This method is obsolete, it has been replaced by System.Configuration!
    System.Configuration.ConfigurationManager.AppSettings”。
    于是就想找到ConfigurationManager类，结果是我引用了System.Configuration，也无法找到，

NorthwindConnectionString;