I made mmf today and just checked it and it is very dry and crumbly. What can I add or do to it to soften it up and make it pliable?

I put mine into the microwave for about 15-20 seconds...makes fondant warm enough to knead a little. Just kept putting back into microwave for this time amount (kneading after each nuke) until it became soft and pliable again. Hope this helps.

also -- work in more shortening a little dab at a time -- easiest way -- rub on palms of hands to grease them up and then knead the mmf.

When I was first learning about making MMF I also made some batches too dry.
I would made another batch and left it sticky, Then kneaded the two batches together in workable amounts. The fondant will last for a long time so if you can't use 2 batches at once wrap it up and store it.
This method worked well for me.

i break mine into smaller flattened batches and rub with crisco and store, as you know you have to grease your hands well before mixing da i didn't do that once ahhhhhh, and also when kneeding your fondant. if it's just to dry for micro you can add a tiny tiny bit of water, add a little at a time crisco is the biggest secret

I also got mine way too dry once, I just melted up more marshmallows and mixed it all together in my kitchen aid. Worked great. It was the best batch I ever made. LOL.