There should be a DMZ (Demilitarized Zone) setting in your web configuration for your router. This allows one computer on your network to be reached outside the network. See if you can find this setting... It'll say something like:
IP Address: 192.168.0.[ ]
And you enter your computer's network IP address in that box and click Enable.
EDIT: This is what mine looks like for my D-Link router web-configuration.
Hope this helps.