add_filter( 'add_to_cart_text', 'woo_custom_product_add_to_cart_text' );            // < 2.1
add_filter( 'woocommerce_product_add_to_cart_text', 'woo_custom_product_add_to_cart_text' );  // 2.1 +

function woo_custom_product_add_to_cart_text() {

    return __( 'My Button Text', 'woocommerce' );

}