Display SKU for variations in MP 2.8

I'm running a small loyalty program website. Right now it works as Marketpress -> Cubepoints.
Customers get points which they can spend on certificates.
These certificates are kept as product variations with codes added to SKU field.
I managed to display certificate code in the order_tracking page by adding the following snippet to template_function.php under function mp_order_status() section
as advised at WMPU

$sku = get_post_meta($product_id, "mp_sku", true);
$sku_var = $sku[$variation];
for ($k=0; $k<count($sku_var); $k++) {
print "<h2>SKU </h2>";
print "<h1> $sku[$k] </h1>";}

As I don't know any of PHP I didn't succeed in adding code for SKU to email template, so I decided to upgrade to 2.8 as it was said to support mp_sku natively.

So here's my problem:
Warning: implode() [function.implode]: Invalid arguments passed in /home/farphor/b.santehplanet.ru/docs/wp-content/plugins/marketpress/marketpress-includes/template-functions.php on line 1838
Tried to use ", " "," ";" as seperator but with no result