I don't know the first thing about an Optimum modem but your OS doesn't detect a cable modem usually. Generally the OS detects your NIC and it in turn finds the cable modem. If you get your i.p. address via DHCP and your NIC is supported it should be quite simple. Open a terminal and enter:

sudo netcardconfig

If your card is supported then it should load the driver for it. To see if you have an i.p.:

ifconfig

If you need to restart networking:

sudo /etc/init.d/networking restart

Post some more details such as NIC model.