Arduino Pro Micro as Usb to Serial Adapter
First you have to burn the USBtoSerial firmware on the Pro Micro. For that you can use for example a UsbTiny or another Arduino (see http://arduino.cc/en/Tutorial/ArduinoISP). You find the hex file on the right side under "Downloads"
For USBtiny you upload the firmware with:
avrdude \ -C<Arduino IDE Path>/hardware/tools/avr/etc/avrdude.conf \ -v -patmega32u4 -cusbtiny \ -Uflash:w:Arduino-usbserial.hex:i \ -Ulock:w:0x2F:m
Now the Pro Micro is ready to be used as a USBToSerial. Howerver you need a 1k resistor and a 100n capicitor for the DTR (to reset the slave board).