Although the other guides do a great job of showing you where to find the civilians, I felt I needed to elaborate a little on a GLITCH that keeps this achievement from popping.
If you play in Co-op, make sure that you rescue ALL the civilians using Player 1, otherwise the achievement may not unlock. Some forum users report that even if you save some of the civilians using Player 2, you can unlock the achievement by replaying the levels and saving the civilians with Player 1, but I can't confirm.
I can, however, confirm that even if this achievement glitches for you, it won't prevent you from scoring the 100% Completion achievement, which unlocks as normal.