How helpful was your post in resolving his situation?
Anyway, you will probably have luck by looking through your routers administrative control panel. More than likely it is under the label of "Port Forwarding" or "Port Range Forwarding" or even "Applications and Gaming." Although the last one is more commonly used with Linksys Routers, it might be used with others. Looking into your manual for the router can help you though.
Once you have found this section, usually you will need to allow Port 21 through your router, and direct it towards your computer that is acting as an FTP Server via it's IP address. Usually there will be a spot to enter the name of what the port is opened for, a range of what the FTP port is, which 99.9% of the time, it's Port 21, and an option to enable your settings.