Starting with version 5, the Tomcat documentation no longer recommends putting Context entries in server.xml.
You don't need to do this anyway. If the directory structure of your webapp is correct, Tomcat will do this for you.
Just make sure you have a 'WEB-INF' (case-sensitive) directory under the root directory of your app.
You should also put a web.xml file under WEB-INF but it's not requried.
Example:
TOMCAT_HOME/webapps/MyApp/WEB-INF
There are some sample war files at
http://simple.souther.us that you can download and drop in your webapps directory. They all have the correct directory structure and a proper web.xml file.