Yes USB should be fine for the power. As for the resistor it will depend on your opto, the ones I use have a 10K pull up resistor built in.

Hood

The more I look at this the more confusing it gets, looking at the schematic for the HobbyCNC Pro board the input pins are all pulled high with a 10K SIP resistor pack, so it is looking more and more like I would not want to draw power from the PC supply and just pick off +5 from the controler board for the LED and wire the phototransistor from ground to input w/o any resistor or perhaps just a few hundred ohms for safety's sake. Last thing I want to do is fry the on-board port, I have a seperate card but It didn't seem to work last time I tried it, Have to check the address and settings again.

Kristin