Should one use resistors to protect an mbed when using it to simulate keyboard key presses
As pjc50 said, you can't put resistors in the power or data of the USB without interfering with the communication. 

If you want to avoid any damage to the USB port, I'd recommend leaving disconnected the VCC pin of the USB breakout board, powering the MBED with a different power source. You will prevent any shortcircuit due to incorrect wiring.