Welcome to the Support Communities!
The article below may be able to help you with this.
Boot Camp Installation & Setup Guide
What you need:
An installation disc with the 64-bit version of Windows 7 Home Premium, Windows 7 Professional, Windows 7 Ultimate, Windows 8, or Windows 8 Pro.
You must use a single full-install Windows installation disc. You cannot use an upgrade version of Windows.
An external drive formatted as MS-DOS (FAT).
I hope this information helps ....
Have a great day!