Well, looking implies a search for something, but there's no guarantee that the search ends. So it could mean that the characters are looking for Alaska both in the physical and the metaphorical sense. Miles wants to know who Alaska is when she is alive in the book, but he never really finds her, and after her death the Colonel and Miles search for an explanation, but they never really solve anything. The looking doesn't really give them peace.
So the title's kind of misleading. The characters only come to terms with Alaska's death when they stop looking for her and accept her as she is and as she was. They only heal after they quit searching for the "real" Alaska and start to seek answers about grief and death in themselves instead.