The USB programmer board allows for easy program upload and communication with the Critter Board. It plugs into the Critter Board programming header.
It utilizes a FT232R USB-to-UART bridge IC providing a serial connection over USB to the Critter Board. Two switches select Program and Run modes. In Program mode the computer is able to reset the LPC2138 into bootload mode. If the USB programmer is just being used as a serial interface to the computer, the switches should be in the Run position.
Make sure that the Critter Board is powered up before attaching the USB programmer board.
More info on using the USB programmer board can be found