If OpenClinica.xml has all the correct settings for your database already, then maybe you can double check your database to make sure connections are getting through to it.
1. Can you run pgAdmin and log into the database independently of Tomcat?
2. Can you review the user_account table and make sure the 'root' user exists?
Also, keep in mind that we issue the OpenClinica.xml file that works with Tomcat 5.5, there may be changes you have to make to accommodate Tomcat 6.
Let us know if that helps, Best, Tom