Put it in the microwave just for a few seconds at a time and knead it. If you have made just one batch the size of baseball and a half, microwave it for 10 seconds. If that doesn't help, microwave for a few more seconds until it works. Let me know if this helped.
Cracking = needs water and some crisco. A little bit of water will go a long way. The micro will soften it, but it won't get out the cracks. The crisco will also soften it, but won't make it smooth like what you want.
The water kneaded in with some crisco on your hands will give it the moisture it needs and the crisco will help lock it in.
Use the water sparingly -- start with less than 1/8 of a tsp -- a few drops really. Knead and then keep adding until it's where you want it. 
Sarah
A couple weeks ago I had the same problem. I microwaved and kneaded in crisco and nothing worked. Someone helped me here. Use 10.5 ounce mini MM and 2cups of confect. sugar tablespoon of Light corn syrup and 1/8 cup of shortening. The fondant turned out perfect! Hope this helps!
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