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Connect it to the coax line and power it up. Let it fully sync up with the system ("DS, "US", and the "Online" lights are steadily lit). Connect a computer to it and open up a browser. You *may* see their "Walled Garden" modem self registration page. If so, follow the screen prompts. Have your account number handy.
If not, try calling 1-855-652-3446. That is their device activation line.
If that doesn't work for you you can call 1-800-comcast and tell a rep that you would like to have a new modem provisioned for service. Have the RF / HFC MAC address and the serial number of it handy when you call.