In a saucepan, bring the broth and butter to a boil. Stir in the couscous. Remove from the heat; cover and let stand for 5 minutes. Transfer to a large bowl; cool to room temperature. Fluff with a fork.
For dressing, in a small bowl, combine the oil, lemon juice and cumin. Add the crab, mangoes, tomatoes and onions to couscous. Drizzle with dressing and toss to coat. Yield: 6 servings.