Hi there. I haven't seen the usb outlets in the new pods but in general the ones I've seen feed 12V to the outlets where it is internally dropped to 5V. If so, the power source would be one of the fuses in the load center. The breakers are for the 120Vac circuits, you can ignore them.
If the usb outlets are dead and no fuses are blown then probably the only way to determine which one will be to remove the usb outlet from the wall and check the voltage on the conductors feeding it with a voltmeter. Pull the fuses one at a time till the voltage is gone, then replace the outlet, reconnect the conductors keeping the polarity the same and reinstall the fuse.