In 1 quart saucepan, heat water and 1/4 teaspoon salt to boiling. Remove from heat; immediately stir in couscous. Cover; let stand 5 minutes. Fluff couscous with fork.
Meanwhile, in small nonmetal bowl, combine all dressing ingredients; mix well.
In large bowl, gently toss cooked couscous, beans, cucumber, olives and green onions. Pour dressing over couscous mixture; gently toss until coated.
Just before serving, add tomatoes; toss gently. Serve warm.