splashingpixels.com on "[Plugin: WooCommerce - excelling eCommerce] Adding custom product tabs to 2.01"

You need something like this...

function my_tab( $tabs ) {
    $my_tab = array( 'my_tab' =>  array( 'title' => 'my tab', 'priority' => 9, 'callback' => 'my_tab_func' ) );

    return array_merge( $my_tab, $tabs );
}

function my_tab_func() {
    echo 'working!';
}
add_filter( 'woocommerce_product_tabs', 'my_tab' );