Grate orange; set orange rind aside. With a small sharp knife, peel off orange pith (the white part underneath the rind) and inner white membranes between orange segments. Chop orange into 1/2-inch pieces.
Place cranberries, 3 tablespoons water, orange rind and sugar substitute in a small saucepan. Bring to a boil over medium heat. Cook 10 minutes, stirring occasionally, until cranberries pop open. Remove from heat; transfer to a bowl.
Mix in orange pieces and walnuts. Chill one hour for flavors to blend.
In order to calculate Net Carbs of any whole food item, it's a simple calculation based off the Nutrition Facts panel:
Total Carbohydrates - (Minus) Dietary Fiber = (Equals) Net Carbs
For low carb products sweetened with sugar alcohols (ie: glycerin, maltitol, etc), refer to the Nutrition Facts panel of the product:
Total Carbohydrates - (Minus) Dietary Fiber - (Minus) Sugar alcohol = Net carbs
This reflects the fact that Dietary Fiber doesn't impact blood sugar levels. Sugar alcohols also have a negligible impact on blood sugar, they too are subtracted from total carbs to yield the Net Carb count.