trouble signing mails with dkim

Hi there,
I’m having trouble signing my outbound emails with dkim.
My DNS record is set and tested fine with http://dkimcore.org/tools/
I just can’t find where to add the dkim signature. Is it class.phpmailer.php ?
i tried to edit this part of the file :
/**
* DKIM selector.
* @var string
*/
public $DKIM_selector = ‘’;

/**
 * DKIM Identity.
 * Usually the email address used as the source of the email
 * @var string
 */
public $DKIM_identity = '';

and so on but didn’t work.

I’m quite new as a webmaster so i mai make obvious mistakes.

Could someone here help me with that i suppose stupid issue ?
Thank you in advance

bump