I am also playing with the Br board. Since I am “prototyping”, I added Br Female Headers (see product page) to the Br board (soldering required) allowing me to easily move wires in my project. The pinout for the Br board is on the Br product page (https://cdn.alchitry.com/docs/Br%20Element%20Reference.pdf). In the constraints file, you need to declare the pins you use. The naming convention is Bank|Pin (e.g., A2, A5, A8, B3, B6, B9).
You also need to be careful with the input and output voltages. I believe the Au is only 3.3v tolerant and has no overvoltage protection on the FPGA pins.