Your mail hosting needs to be active to create new mailboxes.
If you see this screen in the "Mail Hosting" tab, then you can start creating mailboxes:
If you see this screen, then you need to ask your ISP to change your MX records to inc.inbox.com. If you have already done so, you must wait till the records are changed.
If you see this screen after you've activated the mail hosting, or even created addition mailboxes, it might have happened that your domain has expired or the MX records have been changed.