Messages from the emplyer is sent to the mailbox in your Jobseeker account. You'll receive a notification to your e-mail that a message is available in your mailbox. If it is possible to reply to the message, the employer will include a reply-button.
If there is no such button and you still wish to contact the employer, you can find their contact details in the job vacancy text in "sent applications" in your Jobseeker account.
Have you received an application in the message you have problems opening? See more information and solutions here.