If that modem is simply a modem, then you do not need to forward any ports, because there's no routing or NAT being done within the modem. Your computer that is connected to that modem is fully exposed to the internet already and port forwarding on the modem would be useless (and impossible).
If that modem has a built-in router, however, then port forwarding needs to be done.
Look in your System Preferences under "Network" -- does the IP address of the machine look something like either 192.168.XX.XX, or 10.XX.XX.XX, or is it some other number? If it begins with 192... or 10... then most likely it's a router. If not, you're connected directly to the internet and do not need to forward any ports at the modem level.