your callback url should be set in blesta gateways and send via options to your whmcs gateway . something like
Configure::get("Blesta.gw_callback_url") . Configure::get("Blesta.company_id") ."/your_blesta_gateway/?client_id=" . $this->ifSet($contact_info['client_id'])
UPDATE: Just tried this on Safari on my Macbook with no problem everything works fine but it doesn't work on Chrome on the Mac either - I was using Chrome on my Linux install.