Are you supposed to cover your cake in something before you put the mmf on or can you put the mmf right onto the cake???
You icing with buttercream first ... here are some instructions I did that Jackie post in the how to section that will help you 
http://www.cakecentral.com/article47-How-to-Make-and-Decorate-with-Marshmallow-Fondant-MMF.html
Hey shortstuff just incase you don't know this trick. After you get your buttercream icing on your cake, let it sit for about 5mins. this is called "crusting". Get a VIVA paper towel (the reason we say VIVA because it has no designs on it and leaves a smooth finish) If not you could use a reg. paper towel and it will leave neat designs on the cake. But since your doing a fondant cake use VIVA. Anyway once it's crusted lay the paper towel down on the cake and smooth with your hands or with a fondant roller if you have one then lift it slowly..It will leave it so perfect. Give it a try! This will help keep you fondant looking very smooth! GOOD LUCK! 
I also use buttercream, Make sure you smooth it well fondant shows all the bumps lol
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