The first crate is located next to XM8C sub-machine gun, next to the hill with the antenna on top.
The second one is in the barn You reach after destroying the second antenna box and shooting down the helicopter.
Once You clean that barn, go straight towards the south, to a white cross indicating a weapon stash. You'll find a weapon (MG36) and the gold crate in a white building marked with the mercenaries' logo.
The fourth box is in the mercenaries' compound, on some colored containers. You can get there only by jumping from the top of the staircase located behind those containers.
The last crate is at the harbor, close to the cargo ship. When You unlock the last part of the map, enter it, turn right and look for a barrack marked with mercenaries' logo.