Heat oven to 350 degrees F. Coat three 8 x 8 x 2-inch square metal baking pans with nonstick cooking spray.
Mix muffin mix, flour, eggs, juice and oil in large bowl. Fold in blueberries, raspberries and walnuts. Divide evenly among prepared pans.
Bake in 350 degree F oven 25 to 30 minutes, until wooden pick inserted in centers comes out clean and tops spring back when lightly pressed. Cool in pans on wire racks 15 minutes. Invert onto racks to cool.
Transfer 1 layer to cake plate. Spread top with 1/3 cup frosting. Continue layering cake and frosting. Then frost outside of cake completely. Garnish with fresh blueberries, if desired. Makes 16 servings.