Reheat oven to 350 degrees. Prepare cake mix as pkg directs. pour 2 cups batter into greased 13x9-inch baking pan; bake 15 minutes. in heavy saucepan, over low heat, melt caramels and margarine. Remove from heat; add condensed milk. Mix well. Spread carmel over cake; spread remaining cake batter over carmel. Top with nuts. Bake 30 minutes or until cake springs back when lightly touched. Cool.