“System.Configuration.ConfigurationSettings.AppSettings”已过时

在c#里面使用配置文件，出现警告
“System.Configuration.ConfigurationSettings.AppSettings”已过时:“This method is obsolete, it has been replaced by System.Configuration!System.Configuration.ConfigurationManager.AppSettings”

然后添加引用using System.Configuration;
将System.Configuration.ConfigurationSettings.AppSettings修改为System.Configuration.ConfigurationManager.AppSettings

出现错误
错误    1    命名空间“System.Configuration”中不存在类型或命名空间名称“ConfigurationManager”(是缺少程序集引用吗?)

.Net2.0在搞什么飞机？？还是我操作 不对、？