1. Register your licenses, as described in this article.
2. Click Manage your backups and log in using your account credentials (the ones you use to log in to your account on Acronis website):
   
3. Create new Backup Account by clicking on Add:
   
4. Provide information and create a Backup Account:
   
5. Activate the Backup Account by clicking on the activation link you receive to your e-mail.
6. Specify a password for your Backup Account:
   

Now you call install Backup Clients on the machines you want to protect. See Acronis Backup Service: Installing Backup Client.

If you have not received or lost the activation link, resend it using Management Console.