When connecting by MIDI cables to your interface, the interface's MIDI ports should show up in Ableton, but MIDI DIN ports don't "know" what device (Rev2 in this case) is connected to it. But you should be able to find the MIDI ports of your interface, if they aren't disabled or anything like that. I don't use a Mac, but the equivalent to the Device Manager of Windows should help you sort it out. The MIDI ports will most likely be named to reflect the name of your interface in some obvious way.
Concerning MIDI via USB, yes that's a very common cause for Ground Loops, and I would bet (if I had to, as I don't bet) that just disconnecting the USB cable will solve the hum. Try! I know you can't use MIDI at the moment you do this, but you'll get the answer in a second.
I personally, in general, try to use as few USB cables as possible, and go for DIN MIDI to a dedicated MIDI interface as much as possible. Especially when connecting multiple gear that use both audio and MIDI. This because of having had problems with said ground loop issues in the past.
1. Hard to tell, it's like a guessing game. My personal experience of USB ground loops and ground isolators. Works in some cases, in others not. I never figured why or how.
2. The Rev2 itself will not be "recognized" as a unit/interface of its own when using MIDI DIN. Instead you'll be looking for the MIDI ports of your interface. You can probably, however, rename the MIDI DIN ports of said interface to say "Rev2" instead of their standard generic name, once you've found them.