Arduino to Rainbowduino via I2C

After searching countless pages searching for an answer, I've yet to find a guide or at least code to use an Arduino to control a Rainbowduino LED matrix via I2C. If anyone knows how to simply light up the matrix using code on the arduino, please reply. (in the future I hope to wire a controller to the arduino in order to play games on the matrix, but this is difficult if I don't know how to send commands via I2C)

What Rainbowduino do you have? I have a couple of these Seeed Studio Bazaar, The IoT Hardware enabler. and they have the ATmega chip built in so no need to control via a separate Arduino.

I can program straight to the Rainbowduino, but I also want to include and buttons, hence the need for an arduino.