I tried working with chocolate marshmallow fondant yesterday, and it was a disaster. It got rock hard, and nothing I did would soften it up. I finally gave up and threw the entire batch in the trash can. This bring me to an interesting point. There are some things that you can do to MMF to soften it up if it gets hard.
1. Microwave the fondant for around 15 to 20 seconds. This heats it up and makes it manageable. This is also good if the fondant is cold and you don't have time to let it come to room temp on its own.
2. Knead some shortening into it. This helps if the fondant is dry. It's messy, but usually works.
In my case, neither of these worked on my chocolate fondant. I will definitely have to ask someone, who is smarter than I am, why this didn't work. Rest assured that I will post an answer if I get one.