Moles errors in MS Tetsing

Hi Experts,

i have moles in my test method for mock dates. i give reffernces my test project , and run my test method  i got this error 

The host type 'Moles' cannot be loaded for the following reason: The key 'Moles' cannot be found.

please any help me ?

here is my test method :

[TestMethod, HostType("Moles")]
        public void TestPS()
        {
            MDateTime.NowGet = () => new DateTime(2012, 9, 01);

            var inputMap = new XPathDocument("source.xml").CreateNavigator();
            var expected = new XPathDocument("expected.xml").CreateNavigator();
            var actual = _target.TestMap(inputMap);
            XmlAssertHelper.AssertXmlsAreEqual(expected, actual, );
        }

Thanks,

sat