From the instruction PDF...
The LED Limiting Resistor Values in Ohms limits the current to about 12 ma. So it's depending on the voltage...use Volts/12 ma = resistance
For example if you power with 12 volts AC or DC (observe polarity), then 12 V divided by 12ma = 1000 ohms or 1K ohm resistor and rate the resistor at 1/2 watt.