MAX7219 and 51 MCU 8x8 LED matrix

1- Apart from the Power and Ground, I can't find the corresponding pins on the ARDUINO.

I assume that this is still your question.

This bit of the code tells you:-

int dataIn = 2;
int load = 3;
int clock = 4;

So:-
Din -> Pin 2
Cs -> Pin 3
Clk -> Pin 4

Next time use code tags when posting code!