From the looks of the URL that you're using to try to access your servlet, I'd guess that you're using a slightly out of date book or tutorial.
In the past, you could call servlets this way via the InvokerServlet. That has been commented out of the Tomcat config scripts for the last few years. Your servlet will need to be mapped in your deployment descriptore (web.xml).
You are trying to use the InvokerServlet, which is why you can call the servlet without configuring it in web.xml, but the InvokerServlet is not enabled by default, you would have to configure Tomcat to allow this first.
Much better to configure the servlet correctly and not use the Ivoker, it is not a good thing to use.
Sunesh, when that version of core servlets book was written the examples used a previous version of tomcat. You eithr need to access the servlets as the previous posted have mentioned, get a more updated book, or just download the version of tomcat that book recommended, which I believe is version 3.1 or so.
BTW, that is a very good bok to learn from too.
Bosun (SCJP, SCWCD)
So much trouble in the world -- Bob Marley