A cable modem will not show up in device manager. The cable modem is transparent to the network.
Is your account set up with the cable company for high-speed access?
When you connect up the cable modem, all the lights should be on. One of the lights indicates the connection from your NIC to the cable modem. If it is on, the problem is most likely between the cable modem and your ISP. You'll have to call them.