To clarify mis-conceptions: The USB jack is reversible - it works as input and output. Plus the female port makes sense, according to USB specs. If you want to power it thru USB, and you don't have a USB male-male cable, but a (more commonly used) USB A-type male to micro-usb male, use an adapter like this:USB to micro-usb adapter
PLEASE note, that the USB connector is connected in parallel with the DC barrel jack
Of course it is not. The USB is connected to 5V output of the regulator and it can be used as both input and output. When USB is used as input then input voltage should be 5V and barrel connector should be disconnected not to power the 5V regulator. Here is another schematics where the connection is more clear.
Well the one i had, it was in parallel. The switch didn't disconnect the USB-plug. When i put in 9V in the DC-jack it had 9V in the USB-plug.