This Data Pad can be found on Mission 4: 'Tip Of The Spear' on Legendary Difficulty.
Soon as you get into the ghosts (after the first data pad) just follow the road until a drop and park your vechile in front of the beam.
Use your vehicle to jump up the beam and then once again jump to the next platform to find the data pad.
I made this video to help:
Hope this helps.