Yeah, you need to randomly encounter it first so it will register in your Pokedex.
Then, find a house near grass, water, any thing you can encounter Pokemon in. Go in, then out of the house, then check the "Area" of Latias in the Pokedex. If she's not in the area, just repeat the process until she's in your area, then move around in the grass, water, etc. and search for Latias.