From a tutorial I found:
Each mole type is named by prepending an “M” to its name and placing it into a .Moles namespace.
So maybe your line
var stubTestClass = new StubsTutorial.Moles.STestClass();
should be changed to
var moleTestClass = new StubsTutorial.Moles.MTestClass();
though I have no experience with Microsoft Moles.