So i used my usual MMF recipe (16oz marshmallows, 1/4 cup water, 2 lbs PS) Before adding the PS I dyed it to a bright purple. I dumped the ps in and it ended up being too much, but I kneaded it into a ball as best I could and added a couple dollops of crisco. It has tons of bubbles and when I press my thumb in you can see it tearing/breaking. Should I add more crisco? I think part of the problem is I also have trouble kneading such a large hunk of fondant. This is for a cake I'm decorating tomorrow and I'd hate to have to go and buy regular fondant. Any suggestions?! So frustrating :(

Try dividing your fondant into 3 or 4 pieces, knead  a few drops of glycerine into each piece. As you work on each piece keep the rest covered in a ziplock or air tight container. After you have kneaded each piece separately combine all together. HTH 

Thank you! Where would I find glycerine? Is it usually with cake decorating stuff?

I usually get it online @ amazon, but Wilton also has a small size available @ Michaels. Yes in the cake decorating aisle.