The zero is found towards the end of the game and at the start of the mission: operation baby lift.
You start as a helicopter above some paddy fields with the objective in front of you.
The zero is in the paddy fields, slightly behind and to your right.
Stay under 100ft and turn around and go forward and left - it's sort of hidden in the paddy next to some trees but if your low enough you will get the red 'X' and can see some of the white debris from the plane.
Land on the X and wait until it says you have found a WWII era plane. Now complete the section without dying - if you do die, you need to go and pick it up again.
The achievement will unlock when you get back to the carrier at the end of the mission. Good luck!!