Semi-homemade Carrot Cake

Okay, so this is easy and delicious and it tastes like you made it from scratch. (Except I only half made it from scratch!- Which makes this baby SEMI-homemade!)
1 box carrot cake mix + ingredients that it calls for
1 cup shredded coconut

Cream cheese frosting:
8 oz. package cream cheese, softened
1/4 cup margarine
2 1/2 cups powdered sugar
hot water, as needed

Walnuts, crushed

Mix cake mix according to package directions. Stir in coconut. Bake as directed on box.
Allow cake to cool.
Cream together cream cheese, margarine, and sugar in a mixing bowl. Add a little hot water, one teaspoon at a time, until you've reached desired spreading consistency. Spread over cake.
Sprinkle walnuts over top, if desired.