All you need to do is follow the steps outlined here.
The cable modem (or eMTA if you have XFINITY Voice) MAC address is a twelve-character alphanumeric identifier. It is usually found on the back of your cable modem. The MAC address is identified by one of the following prefixes:
- CABLE RF MAC
- CM MAC
- HFC MAC
- RF MAC
Depending on the type of device, you may see multiple MAC addresses. In this example, the cable modem MAC address is listed on top (the "RF MAC").