You'll have to tell the router to open ports 6891 - 6900 for incoming TCP connections, and also tell it to forward connections to those ports to your machine. Exactly how this is done depends on what sort of router you have. You'll have to check the manual, or have a google around to see how it's done for your make and model.
If you happen to have a Netgear router, then I can take you through it step by step. If not, then you'll have to find out yourself (I'm sure someone may be able to tell you how to do this in the 'Hardware' forum).