Interfacing an optoisolator with arduino
According to the schematic, there are current limiting resistors in place (R1 and R2) so you can just connect your Arduinos outputs directly to IN1 and IN2, and it's ground to GND.  

The rest of the connections are for the isolated side (marked "noisy system" in the schematic) 
so you connect your separate power supply (the one you want isolated from the Arduino) to HV (V+), and HVG (ground - not the same ground as Arduino) and the signals come from OUT1 and OUT2. If your isolated supply is e.g +15V, then your signals will be from ~0V min to +15V max (assuming digital signals)