In a 1-1/2-quart slow cooker stir together sweetened condensed milk, ice cream topping, and chocolate.
Cover and cook on low-heat setting for 2-1/2 to 3 hours or on high heat setting for 1-1/2 hours, stirring once halfway through cooking. If no heat setting is available, cook for 1-1/2 hours. Stir until chocolate is melted. Serve immediately or keep warm in the cooker on low-heat setting (if available) for up to 1 hour. (Chocolate mixture may become grainy if held longer.)
To serve, spear dippers with fondue forks. Dip into chocolate mixture, swirling as you dip. If the mixture thickens, stir in milk to thin. Makes 12 servings.