Once you need to get into the Grand Hotel, you need to speak with a couple of laundry workers. They'll give you a keycard if you do their work for them. Take their car to the three different laundry containers in town, collect a box from each container and bring it to the back of the car, and drive to the Ethan's Bar. Talk to the workers inside, and the achievement will unlock.
This is a story achievement, and can't be missed during a playthrough.