Enterprise, a few quick googles and I found that your "modem" is a router-modem.
It is physically impossible to port forward on a straight modem (unless using a dial up connection where extremely little work is required). A normal modem contains no kind of firewall equivilent.
Please don't help if you don't know what you're talking about.
Annoying Orangeet I would suggest following Wedges link if you are sure you have port forwarded correctly, it is a very good guide.