Preheat oven to 350 and place rack in the center of oven.
Mist a 13" x 9" metal baking pan. Dust with flour and shake out excess.
Place the cakemix, water, oil and eggs in large mixing bowl. Blend with mixer on low speed until ingredients are incorporated (30 seconds). Scrape the sides of bowl and increase speed to medium and blend for 2 minutes or until the batter lightens.
Pour the batter into the pan, smooth the top and place pan in oven.
Bake the cake until the center springs back when lightly pressed with your finger, about 30-35 minutes.
Remove cake from oven and let rest for 15 minutes.
For the glaze, combine the lemonade concentrate and confectioners sugar until smooth. Poke a long-tined fork into top of the warm cake every 1/2", wiping the fork occasionally to remove crumbs. Drizzle the glaze over the top of the cake.
Cover the pan with plastic wrap and place it in the refrigerator to chill for 2 hours. Remove pan from frig, spread the whipped cream over the top and slice and serve.