Hi,
It depends mostly where you want to display it.
HikaSerial have views for the content it adds in the pages or emails, like "orderserial / show_email_notification_html" for the notification email (one in the front-end and another for the backend dependings from which content the mail is sent).
I have made a patch for HikaSerial which will be in the next release ; the patch gave the "extra data" unserialized in the view.
For the moment, in the view, you need ot add that code
if(!empty($serial->serial_extradata) && is_string($serial->serial_extradata))
$serial->serial_extradata = unserialize($serial->serial_extradata);
in order to unserialized the extra data to be sure to have an array and not the serialized string.
Then you can display the extra data you want, like
echo $serial->serial_extradata['mykey'];
Regards,