To retrieve Khalim's Heart, you must first make your way to Lower Kurast (Waypoint) which is located past Flayer Jungle. Make your way through Lower Kurast until you reach Kurast Bazaar (Waypoint). The Sewers under the Kurast Bazaar are near a staircase. You should see the purple entrance on your mini-map. Khalim's Heart is located on Level 2 of The Sewers.