As mentioned in the Boot Camp Beta FAQ article, you should install the latest firmware updates available for your Intel-based Mac.
You might need to download and install more than one firmware update before you can use Boot Camp Beta depending on which Mac you have. For example, with an iMac (Early 2006), installing only the latest SMC firmware update may not be sufficient--the latest EFI firmware update may also required. Be sure to install all of the appropriate firmware updates for your computer. You can obtain the latest EFI and SMC updates for your Mac by visiting http://www.apple.com/support/downloads .