C# – Where to find “Microsoft.VisualStudio.TestTools.UnitTesting” missing dll

I am getting following error in my C# visual studio project:

The type or namespace name 'VisualStudio' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)

I also tried to find the microsoft.dll file but couldn't get any reference.
Am I searching the wrong DLL?

using Microsoft.VisualStudio.TestTools.UnitTesting;  
using Kya.MsFx.Services.Swiper;

namespace Kya.MsFx.Devices.Swiper.Test
{
[TestClass]
public class SwiperWindowTest
{

    private SwiperWebServiceHost m_SwiperWS = null;
    /// <summary>
    ///     start web service on a separate thread, so tests 
    ///     can be executed withut blocking the application thread
    /// </summary>
    [ClassInitialize]
    public void SetupSwiperTests() {

        m_SwiperWS = SwiperWebServiceHost.StartService();

    }

    /// <summary>
    /// Stop service started during class initialize and kill the thread
    /// </summary>
    [ClassCleanup]
    public void CleanupSwiperTests() {
        m_SwiperWS.Stop();
    }


    /// <summary>
    ///   simulate init, swipe, clear operations
    /// </summary>
    [TestMethod]
    public void TestSwiperService()
    {
        MessageBox.Show("test");
    }
}
}