I'm surprised it didn't worked. I managed to forward a port from a real IP to a VPN station, by adding those two iptables rules to the existing firewall script.
Did you specifically check that the rules were written syntactically correct, and that they can be seen with "iptables -t nat -L" ?
What's the default policy of iptables, on your Ubuntu system? (ACCEPT or DENY/DROP)
For now, I think that checking the stuff i've written above could be helpful.
If the stuff it's correct, and forward still dont work maybe you should begin traffic analysis.
For this, I recommend the following tools: tcpdump(it's a command line tool) or wireshark(aka ethereal), which is a GUI tool.
And, as an alternative solution for forwarding, you can use ssh, or putty. There are tutorials on the Internet about this topic.