Sometimes Marshmallow Fondant will get dry. You can add some Glycerine to fix it. This tutorial shows how to do it. Another way to fix MMF, is by adding pre-bought Fondant to it. I have used it like this when I don't have enough regular fondant and I need to extend it. It works really well too. 

Check for the Marshmallow Fondant Recipe and the
White Chocolate Fondant Recipe.