This achievement will come over time. The defibrillator will spawn randomly around the levels. Once you find one it will show up where your medikit is stored.
Press to equip defibrillator while near one of your dead team mates. Aim at the dead survivor and press the to bring them back to life. This has to be done ten times for the achievement.
You cannot shoot down your own AI or team mates and bring them back to life for this achievement. Either Infected or Special Infected must kill the other players.
See "THE QUICK AND THE DEAD" for an alternative technique.