In a well-greased 13x9-in. baking pan, layer the walnuts and berries; sprinkle with sugar and gelatin. In a large bowl, combine the cake mix, eggs, water and oil; beat on low speed for 30 seconds. Beat on medium for 2 minutes. Fold in marshmallows. Pour over top., Bake at 350° for 35-40 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before inverting onto a serving platter. Refrigerate leftovers.