WooCommerce - Add Custom Product Data Tab

woocommerce-add-custom-product-data-tab.php

// First Register the Tab by hooking into the 'woocommerce_product_data_tabs' filter
add_filter( 'woocommerce_product_data_tabs', 'add_my_custom_product_data_tab' );
function add_my_custom_product_data_tab( $product_data_tabs ) {
	$product_data_tabs['my-custom-tab'] = array(
		'label' => __( 'My Custom Tab', 'my_text_domain' ),
		'target' => 'my_custom_product_data',
	);
	return $product_data_tabs;
}

// Next provide the corresponding tab content by hooking into the 'woocommerce_product_data_panels' action hook
// See https://github.com/woothemes/woocommerce/blob/master/includes/admin/meta-boxes/class-wc-meta-box-product-data.php
// for more examples of tab content
// See https://github.com/woothemes/woocommerce/blob/master/includes/admin/wc-meta-box-functions.php for other built-in
// functions you can call to output text boxes, select boxes, etc.
add_action( 'woocommerce_product_data_panels', 'add_my_custom_product_data_fields' );
function add_my_custom_product_data_fields() {
	global $woocommerce, $post;
	?>
	<!-- id below must match target registered in above add_my_custom_product_data_tab function -->
	<div id="my_custom_product_data" class="panel woocommerce_options_panel">
		<?php
		woocommerce_wp_checkbox( array( 
			'id'            => '_my_custom_field', 
			'wrapper_class' => 'show_if_simple', 
			'label'         => __( 'My Custom Field Label', 'my_text_domain' ),
			'description'   => __( 'My Custom Field Description', 'my_text_domain' ),
			'default'  		=> '0',
			'desc_tip'    	=> false,
		) );
		?>
	</div>
	<?php

}

You've got 'add_my_custom_product_data_fields' in add_action but the function name is 'add_mailchimp_product_data_fields'. Works a treat once that's rectified ;-)

Probably me being an idiot, but just in case anyone else has the same problem as me; by "Id below must match target registered in above add_my_custom_product_data_tab function", this means the 'target' parameter, not the '$product_data_tabs['my-custom-tab']' part. (Thought there was another error like above till I spotted target!)