I am trying to enable "Back to My Mac" on my desktop computer so that I can reach it when I am traveling with my laptop. I have a SMCD3G-CCR modem. The Mac documentation says that I need to have Universal Plug and Play enabled on the modem - however, when I log in to the modem I don't see any tab or setting that accomplishes that.
Unfortunately, the SMC gateway does not support UPnP or NAT-PMP, either of which are, apparently, required for Back to My Mac. The Cisco DPC3939B, however, DOES support UPnP. So, what you could do is either call in to 1-800-391-3000 and request an upgrade to the Cisco gateway (be aware there is a ~$50-$99 charge for installation), OR you could go out and purchase a standalone router that supports UPnP or NAT-PMP, and connect that in between the SMC and your Mac.
As a last resort, Apple has a VERY long list of all of the ports used by their software located here
You coudl TRY going through that list, and setting up port forwarding rules on the SMC for each port that Apple says Back to My Mac requires. Be warned, there's a LOT of them (i'm seeing UDP ports 1900, 4488 and 4500, and TCP ports 5223, 5350 and 5351, but there are probably more in the list).
The standalone router option will probably be your best bet.