Center ADD TO CART woocommerce shortcode

without a link to the site there will be no chance to help you.
if there was a hidden link – than you have to wait for the mods.
Sorry

if it is the same thing like here: http://kriesi.at/themes/enfold-shop/product/casual-collection/

important for that is to give the whole form.cart a width – otherwise the centering will be not so easy

form.cart {
    width: 300px;
    left: 50%;
    transform: translate(-50%);
}

button.single_add_to_cart_button {
    width: 160px !important;
}