The correct way to execute a Servlet is to map it to a URL and then access it through that URL eg /venu/MyServlet Accessing servlets as /venu/servlet/ClassName is a horrible old thing that they are trying to get people away from. By default it is disabled, but following the instructions above you can enable the invoker Servlet in Tomcat which allows you to access Servlets in this way.
The invoker Servlet is a special Servlet mapped to /venu/servlet/*, looks at what you asked to invoke and calls it. The big problem is that if you forget to turn it off it allows direct access to all servlets as long as the client knows the servlet class name. This can be a security risk or cause instable applications when used in production, but is OK when learning, as long as you learn how to map Servlets soon!