Hi,
you definetely should be able to join accessible hosts (at least with properly configured or disabled firewall).
Make sure you are trying to connect via an external IP (see for example http://www.whatsmyip.org/);
if your router supports UPnP and it's disabled, you can try to enable it as an alternative to port-forwarding;
also you may make sure that the external IP reported in the router/modem status info is the same as one on http://www.whatsmyip.org/ (someone reported he had ISP's router providing network for multiple flats in the building, so his own router had a LAN-type (192.168-something) external IP, while the IP reported by whatsmyip belonged to the ISP's router).
If you checked everything above and it doesn't help, try to expiriment with someone who is able to both host/join games (you should be able to find such people in the lobby).