OK found the datasheet for it
this is the pinout....
so pin 2 goes to ground, pin 1 goes to your signal via a resistor
The LED's in opto-couplers are reasonably standard ones so
use a ~ 20mA as a max current through the LED, work out the resistance needed
considering the voltage from your signal source ( that transistor to the right ????)
Again PLEASE describe exactly what you are trying to achieve
EDIT .... just realised the signal source is the Arduino so are you trying to use it and the opto to switch the power on and off from the plughpack ? You still need to consider the output from the Arduino and if it is going to supply the LED with excess current .... I dont know anything much about Arduinos
Dave