I have decorated cakes for more than 20 years and have only ever bought my fondant icing. I think that most cake decorators buy their fondant icing rather than make it themselves. It can be tricky to work with depending on the weather, so it's easier to buy it ready made. Here is a link for making it yourself, but it isn't a thermi method.[http://thecookduke.com/fondant-recipe//url]
I have made the marshmallow fondant before, but haven't done it in the thermi, although I will try tenia's method next time. I find that the marshmallow fondant is eaten by children, but the ordinary fondant gets left on the plate.
It may be possible to follow Tenia's instructions using the ingredients in the linked recipe. I would love to know how you get on!