CMC stands for carboxymethyl cellulose. It's a powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.

CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.
I usually leave the body and the head of a cake topper to dry overnight. I attach the head to the body once both the head and the body are very dry or completely dry. If you want to find out more about how to make cake toppers with large heads click here.