/templates/loop/add-to-cart.php

<?php
/**
 * Loop Add to Cart
 *
 * @author 		WooThemes
 * @package 	WooCommerce/Templates
 * @version     1.6.4
 */

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

global $product;

if ( ! $product->is_purchasable() ) return;
?>

<?php if ( ! $product->is_in_stock() ) : ?>

	<a href="<?php echo apply_filters( 'out_of_stock_add_to_cart_url', get_permalink( $product->id ) ); ?>" class="button"><?php echo apply_filters( 'out_of_stock_add_to_cart_text', __( 'Read More', 'woocommerce' ) ); ?></a>

<?php else : ?>

	<?php

		switch ( $product->product_type ) {
			case "variable" :
				$link 	= apply_filters( 'variable_add_to_cart_url', get_permalink( $product->id ) );
				$label 	= apply_filters( 'variable_add_to_cart_text', __( 'Select options', 'woocommerce' ) );
			break;
			case "grouped" :
				$link 	= apply_filters( 'grouped_add_to_cart_url', get_permalink( $product->id ) );
				$label 	= apply_filters( 'grouped_add_to_cart_text', __( 'View options', 'woocommerce' ) );
			break;
			case "external" :
				$link 	= apply_filters( 'external_add_to_cart_url', get_permalink( $product->id ) );
				$label 	= apply_filters( 'external_add_to_cart_text', __( 'Read More', 'woocommerce' ) );
			break;
			default :
				$link 	= apply_filters( 'add_to_cart_url', esc_url( $product->add_to_cart_url() ) );
				$label 	= apply_filters( 'add_to_cart_text', __( 'Add to cart', 'woocommerce' ) );
			break;
		}

		printf('<a href="%s" rel="nofollow" data-product_id="%s" class="add_to_cart_button button product_type_%s">%s</a>', $link, $product->id, $product->product_type, $label);

	?>

<?php endif; ?>