CMC stands for carboxymethyl cellulose. It's a fine 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.
How to use CMC?
Add it shortly before forming the parts. Don't let the paste sit for long after adding the CMC, otherwise it might become crumbly and it will be harder to model and get a smooth finish.
Keep on adding it gradually until the paste gets thicker. Knead the CMC into the paste well - you will feel under your fingers that the paste became thicker (you will need to use more pressure to knead it).