In a medium bowl, squeeze the juice from one lemon. Whisk in 4 tablespoons of oil on a slow, steady stream until well blended. Add salt and pepper to taste. Stir in arugula, cucumber, and mint. Set aside.
In a small saucepan, bring the broth to a boil. Set aside.
In a medium saucepan, heat 1 tablespoon of oil over medium heat. Stir in the couscous making sure the oil coats it. Add the hot broth and salt to taste. Allow the mixture to come to a vigorous simmer, stirring frequently. Remove from heat, cover, and let stand till the liquid is absorbed, about 8 minutes. Fluff with a fork and then add the arugula mixture. Cover and let stand until the arugula wilts, about 2-3 minutes. Serve sprinkled with pine nuts.