Just login to your online account and download the installer from there. You can't use your activation key twice - you already have an active online account.
As Safeboot says, your software will always be available for download on your online account page: Antivirus Software and Internet Security For Your PC or Mac | McAfee
When you formatted I assume you didn't uninstall the software first so it's just possible that the license will be used up, but don't worry, Customer Service can free that up for you:
Best to phone them (free of charge) Services and Support for Home and Office the UK line is open right now.
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Moved this to "My Account Online" as that's basically what it's about.