The silver side of the blanket will go towards the cat, however there are some important points to keep in mind when using a mylar blanket. First of all, the mylar blanket should not go directly on the skin. Mylar blankets will only reflect radiated heat and putting the blanket right on the skin will transfer body heat through conductivity instead of radiation. This will conduct your cat's heat instead of reflecting the heat back to your cat. You need to have some air space between the cat and the mylar blanket. You can do this by putting a thin layer on top of the mylar blanket.
Another important fact to keep in mind is the reflective insulation (of the mylar blanket) will not do much good if the cat is lying directly on the ground. The ground absorbs body heat faster than air. You need to have an additional pad or some other solid insulation between the bottom of the mylar blanket and the ground.
Also, it's useful to know this advice is not only effective for keeping your cat warm, but for keeping dogs warm in the winter, as well as keeping humans warm.