The search functionality in WooCommerce doesn't search by sku by default.\
This simple plugin adds this functionality to both the admin site and regular search.\
Tested with WooCommerce 1.5.6, 2.0.7,2.0.18, 2.1.12
For more info
http://www.codenterprise.com/corporate-profile/wordpress-plugins/
Other WooCommerce Plugins at
http://www.codenterprise.com/our-services/woocommerce-web-development/
Send your problems to info@codenterprise.com. Dont forget to mention WP and WooCommerce versions.