Configuring pound is one of the easiest thing. Let us say your run pound on 190.1.5.6 public ip. All you have to do is
1) stop Apache on 190.1.5.6 port 80/443
2) configure pound to run on 190.1.5.6 port 80/443
3) configure pound to redirect traffic to internal hosts
4) my guess DMZ firewall is blocking something soemwhere
Try following tutorial
Tuple Shop: Deploying Rails with Pound in Front of Mongrel, Lighttpd, and Apache