This achievement will unlock after the first chapter of episode three.
*** Spoiler - click to reveal ***After choosing to either leave the girl in the street or mercy kill her, Kenny and Lee will run into the pharmacy, the loading screen should trigger the achievement.
I have noticed some achievements in this game, along with other Telltale games taking considerable time to unlock. If this doesn't unlock straight away just continue playing and it will unlock after a short period of time.