In a bowl (or a mixer) beat your cream cheese well
On the stove in a small saucepan melt your marshmallows in your milk over medium heat. Make sure to stir often and it should take about 7 minuets. Remove from heat and let cool
Once your marshmallows are cooled, combine them with your cream cheese and beat to combine
Fold in your whipped topping and pour your mix into your prepared crust.
Refrigerate a few hours to set and serve!