Where to place a resistor.

Other Hardware General Electronics
5

MAS3:

arduino_obc_and_rbc:
... to protect the LED

It will protect the LED, that is true.
But the main reason for using the resistor should be to protect your Arduino.
An Arduino pin is limited in the current it can output (absolute max of 40 mA.)
You should stay away as far as possible if you can, and that's the most important reason to use a current limiting resistor.

And you assume he is using an Arduino?