I have not used this device personally, but i believe it is just a standard eMTA; it is intended to be connected to a standalone router, which you would then perform port forwarding on. If it DOES have such a capability, the interface would be accessible at usually http://192.168.100.1 or http://10.1.10.1
We would need some more information as to which port you would like open, which protocol(s) to be used, and to which destination inside your LAN.
The ARRIS TM608G is ONLY a dedicated telephone modem and does NOT support any Comcast Internet service whatsoever. You should be gaining your internet access from ither an SMC8014, SMCD3G, Netgear 3000, or DPC3939B Comcast Gateway (CG). Could you please let us know which of these CGs you are using.