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Boot Camp isn't compatible with Windows 8 at the moment, but if you have OS X Mountain Lion, Apple should release an update soon with Windows 8 support.
If you want to use Windows 8 now, you can install Parallels, VMware Fusion or VirtualBox and install Windows 8 on a virtual machine. Another option is to install Windows 7 on Boot Camp if you want.
If you need help installing Windows, follow the steps > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.7.pdf