Place frozen meatballs in the bottom of a slow cooker or crock-pot. Turn slow cooker to low.
In a medium saucepan combine ketchup, tomato soup, grape jelly, grape juice, horseradish, dark brown sugar, cayenne pepper and the juice and zest from one orange. Heat to a boiling, stirring constantly. Remove from heat, cool 10 minutes and pour sauce over meatballs.