If you are using Tomcat 6 (which implements Servlet 2.5 specification) or a newer version, you don't have to change the web.xml in the CATALINA_HOME/conf/ directory to display the directory listings. Instead you should change the web application's own web.xml file under WEB-INF.
As Adarshr mentioned, this is what you need to add to the web.xml
<servlet>
<servlet—name>default</servlet—name>
<servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>0</param-value>
</init-param>
<init-param>
<param-name>listings</param-name>
<param-value>true</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
You also need to add the following
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>