I had a bit of trouble installing the Mango HMI on my CentOS box, but now I've solved it. It's a simple permissions problem, exacerbated by poor error messages in the catalina.log file on tomcat6. Googling the problem just turned up lots of other frustrated users, so I thought I'd wade in with the solution.
Here's the beef: You look in /usr/share/tomcat6/logs/localhost*.log. In there you'll see that the default Derby database didn't have write permission to /var/lib/tomcat6 and /etc/share/tomcat6/. You can find out who is running tomcat easily:
ps aux | grep tomcat
That tells me that "tomcat" is running tomcat. OK. So change the ownership of the two directories to "tomcat" and now restart tomcat6 and Bob's your uncle.