我在代码中 使用 using System.Timers;添加Timers类，但是编译却出现错误，
F:\Program_Files\Microsoft Visual Studio .NET 2003\mywork\ConApp5\Class1.cs(4): 类型或命名空间名称“Timers”在类或命名空间“System”中不存在(是否缺少程序集引用？)如附件所示

不知道除了在代码中这样编辑之外，还要在哪里设置才能正确添加呢？

// page 273

using System;
using System.Timers;   // requied System.dll

//namespace ConApp5
//{
    class Clock
    {
        static int iStringLenth;

        [STAThread]
        static void Main( )
        {
            Console.WriteLine("Press Enter to end program");
            Console.WriteLine();

            Timers tmr = new Timer();
            tmr.Elapsed += new ElapsedEnentHandler(TimerHandler);
            tmr.Interval = 1000;
            tmr.Start();

            Console.ReadLine();
            tmr.Stop();
        }

        static void TimerHandler(object obj,ElapsedEventArgs eea)
        {
            Console.Write(new String('\b',iStringLenth));

            string str = String.Format("{0} {1}",eea.SignalTime.ToLongDateString(),
                                eea.SignalTime.ToLongTimeString());
            iStringLenth = str.Length;

            Console.Write(str);
        }
    }
//}