Thank you for pointing that out!
Not sure how it got omitted.
The part you're looking for is here:
https://www.sparkfun.com/products/10007
I updated the bill of materials to include it.
The shield uses pins 0 and 1 (UART Serial1) to communicate with Bpod. If you use Arduino Leonardo, you can use USB simultaneously. This allows you to change parameters on the module between trials with MATLAB, and trigger the module from the state machine during a trial. If you use Arduino Uno, pins 0 and 1 are routed through an FTDI emulator IC, to create the USB serial connection - so the board would have to be unplugged from the computer (powered using a barrel jack or USB accessory charger) in order to work.
I hope this helps.