I moved this provisionally to My Account Online, because that's where you need to go to check if there's a spurious machine occupying that last license.
You may or may not be able to remove it yourself depending on the type of account. Account is here: https://home.mcafee.com/secure/protected/login.aspx?
If not contact Customer Service by phone or online chat, it's free to do so. Here: http://service.mcafee.com/CustomerService.aspx?lc=2057&sg=CS&pt=1