Add SKU to invoice – PDF Invoices and Packing Lists

I am trying to add the SKU to invoices (using PDF Invoices and Packing Lists)

I can add the extra column to the invoice no problem but I can’t get it to call the products SKU to print onto the invoice.

I’ve tried

$product->get_sku()

But it doesn’t display anything.

What am I missing?

http://wordpress.org/extend/plugins/woocommerce/