Ok, I'm a little late to the party, but I might have something to contribute.
In order to host a game when behind a firewall, you'll have to do the following:
1) Get your router's IP address. That is the IP address that everyone else on the Net has to use in order to connect to your computer, regardless of what your computer's IP address is. You can usually get the IP from the router's configuration screen, but if not, this site will tell you what your router's IP is: http://www.whatismyip.com/
2) Give your IP to your buddy who wants to join the game you're hosting. This is the IP he needs to give to CM.
3) Set your router to forward all packets addressed to port 7023 to your machine's local IP address (it's probably still 192.168.1.34). You may need to reboot the router after this change. Read the instructions carefully.