This is a story related achievement and cannot be missed. You get it in the mission THE HOUSE OF PLEASURE.
The missions are:
Returning Home
Dishonored
High Overseer Campbell
House of Pleasure
The Royal Physician
Lady Boyle's Last Party
Return to the Tower
The Flooded District
The Loyalists
The Light at the End
Once you've entered the Golden Cat brothel, head to the top floor. She's in the second room on your right. (Her location is revealed either through a note in Madame Prudence's office or by eavesdropping on two courtesans in a bathroom.)
The achievement should unlock upon Corvo and Emily's reunion.