Sometimes, for whatever reason, if there is tons of color in it, especially dark colors, it can get crumble. My only suggestion is tons of crisco and microwave it...I ended up throwing some burgundy stuff away after making accents with it. It was gonna be covering a cake but there was no way I was ever gonna roll this stuff out....!!!
Dry and crumbly (that is not fixed by crisco) usually means you need to knead some water into it to soften it. You may need to microwave it a bit to soften it enough to work the water in.
I had this same problem, I ended up buying some premade fondant just to see what I can do with it. Someone here suggested adding glycerine to it to give it some elasticity. I have yet to try it but I just bought some tonight. Also, it might just have to much sugar in it. Try adding some more marshmellow.
Dry and crumbly (that is not fixed by crisco) usually means you need to knead some water into it to soften it. You may need to microwave it a bit to soften it enough to work the water in.
Oh yeah, I forgot about the water, that worked for me to get a nice emerald green fondant for my green rose cake. I added so much wilton coloring it ruined the fondant and water was the only thing that worked.
I had this problem when I made navy blue MMF. Nothing would work and I was going crazy until I decided to mix it with Wilton fondant. I did a 50/50 mixture. I also added Lorann oil while flavoring to mask Wilton's taste. Well, it turned out pretty good! I got the flavor of MMF and the Loran oil and the texture of Wilton. HTH
I had crumbly MMF once and was so frustrated, only to realize later that I had used a 10 oz package of marshallows on accident!
I had this problem too because for some reason, they do not sell 16 oz bags of mini marshmallows anywhere near me. I had to adjust the recipe for the 10.5 oz bags. Grrr.... 
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