Well firstly you have to find out what port the game is using and then install Darrell May's Port Forwarding module from:
http://myezserver.com/downloads/mitel/contrib/portforwarding/Be sure to choose the 5.6 version and then setup the forward in your server-manager, easy as! When I did it I didnt know wether to use UDP or TCP, so i just set it to forward both ports to my internal machine:
Example:
SME ip 123.456.789.001
Internal PC ip 192.168.0.1
Game port req 16000
Setup port forward on SME to forward all TCP 16000 to 192.168.0.1 16000
Setup port forward on SME to forward all UDP 16000 to 192.168.0.1 16000
And bob's your uncle!
I have several ports forwarded for certain games I play plus certain p2p programs that I use and it works great! Thanks go out to Darrell May for the great conrtib!!
Cyrus Bharda