Glenny, It's best to keep one topic in one thread. In your other thread, I gave you a link to the 5.5 JNDI documentation. Now I can see you're using 5.0. Your link points to the 4.1 configuration page.
Also, this is a servlets forum. There is a forum just for Tomcat on this site. If you ask your configuration questions there, you are much more likely to have it seen by someone who has dealt with the same issue.