For Crust: Mix pretzels, sugar and melted butter together. Bake at 375° for 7 minutes. Cool. For Cream Cheese Mixture: Cream together the cream cheese and sugar. Fold in Cool Whip. Pat onto crust. For the Topping: Mix Jello with boiling water. Stir until dissolved. Add frozen berries. Let stand until set. Put on top of cream cheese layer. Refrigerate.