If you open your clay and find it crumbles you need to condition it. Polymer clay contains a plasticizer which can be absorbed by other materials or dry out. What we need to do is to add more of that plasticizer back into the clay.
There are two ways of conditioning the clay, the hard way or the easy way. The hard way is a lot of elbow grease and really sore hands. You will need to work the plasticizer into the clay by kneeding and mixing it, just like dough.
Let's assume you'd like to do it the easy way. The first thing you will need is a small food processor, you can get one for under $20.00 from stores like Walmart. Break your clay up into small pieces and put it in the food processor.
Add a few drops of plasticizer to the mix, but only a few, if it isn't enough you can always add more. It's harder to try to get it out.
Blend the clay until it is really small. The better it is blended the easier it will be. You will now take all of it out and roll it into a ball, flatten it between your hands. You are going to run it through your pasta machine to get it to fully condition. You will end up with clay that has tattered edges.
Run it through the pasta machine over and over and over again. Keep doing this until you have smoother edges, then your clay will be conditioned.