This achievement will unlock at the end of chapter six of episode three.
*** Spoiler - click to reveal ***After you've had the lovely job of deciding who should kill Duck, you'll head on back to the train, talk with Chuck, have some short interactions with Clementine, teaching her how to shoot and then cutting her hair. To get the map, give the bottle of Whisky to Chuck, then head back inside and tell Kenny that Chuck has Whisky. When he has moved out of the way, take the map, and head back to Clementine. The train will stop, and the achievement will unlock.
The achievements in this game can be glitchy at times. If an achievement doesn't unlock then the best thing to do is to continue playing the game, the achievements should unlock a short while after.