Untitled

Why can I not use Tuple in C#, Visual studio 2010?
using System;

namespace TestTuple
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var mytuple = new Tuple<string, string, DateTime>("Alexander", "Moor", Convert.ToDateTime("12.03.2009"));
            Console.WriteLine("{1},{0} born {2}", mytuple.Item1, mytuple.Item2, mytuple.Item3);
        }
    }
}