When you wake up at Mo's head to the trailer towards the left of the map. When you get there the trailer is locked and the owner, Todd, is downstairs in his basement.
You can enter now by kicking the door down, or you can bring Todd up and kick the door down to knock him out. Either way you will end up inside the trailer.
While inside Todd's trailer there are cupboards on the left side of the screen. If you have already knocked Todd out then this cupboard will be above his head.
On the inside of the door of this cupboard is a lockpick. Take this.
Head to the gas tower near the top of the map, and use the lockpick on the padlocked door. This achievement will then unlock.