I started making marshmallow fondant for decorations on my cakes. I thought maybe I wasn't rolling it out thin enough. But it just doesn't harden and dry no matter how long I let it sit out. Any advice?

In my experience, marshmallow fondant never really hardens. You say it's for decorations right? even if you add tylose/cmc it still would not harden. The outside surface would only form a stiff shell but the inside would still be gummy. I suggest using a different type of fondant or use gumpaste, they have lots of tutorials here at CC.

if your going to do homemade fondant, then try Michele Fosters Fondant. It will work, and the recipe is here, and it's inexpensive also.