If you have not put your database's JDBC driver file in Tomcat's common/lib directory, as instructed, you get the message "Couldn't create addressesDB sttribute: Couldn't open connection to Public database: Cannot load JDBC driver class 'org.gjt.mm.mysql.Driver'". So I don't think this is your problem, but please check it anyway.
Otherwise, you can view your Tomcat configuration by pointing your browser at http://localhost:8080/admin and looking at AddressBook's Context and Data Source Panels [see figures 8 & 9]. Do these panels look correct?
You do not need to reference the context.xml in the web.xml. The information in context.xml gets put in Tomcat's conf/server.xml file automatically by the Ant/Tomcat admin process. Check server.xml out with your favourite text editor. Does this look correct?
Finally, what versions of OS, MySQL, JDBC driver, Tomcat and Ant are you using?