Do you have a potentiomter and a multi-meter?
Use the POT with pins 1 and 2 as your resistor. Twiddle the pot until it looks bright enough. Then measure the resistance between pins 1 and 2, pick a resistor that is roughly that value.
To check if it's going to pull too much current, with the potentiometer in place, wire the DMM in series with the pin on mA range.
V+ --> Meter --> POT --> Backlight PIN
Then turn the POT all the way down, but do not exceed around 20mA. If it goes higher than 20mA you will need a resistor. If it doesn't it probably already has one in it.