The ASP.NET MVC djinns must be informed to use your custom validator class when doing validation using your custom validation attribute.

global.asax.cs

public class MvcApplication : System.Web.HttpApplication
{
  protected void Application_Start()
  {
    // ... code omitted ...

    DataAnnotationsModelValidatorProvider.RegisterAdapter(
      typeof(MkayAttribute), 
      typeof(MkayValidator));
  }
}