Read Instructions Carefully
Patched OSInstall.mpkg and OSInstaller.framework to allow OS X 10.10 Public Beta installation on MBR partition.
To use those patched files you must create an USB installer. You can't just run the app downloaded from Mac App Store.
After creating the installer, copy those files like that:
OSInstall.mpkg to /System/Installation/Packages
OSInstaller.framework to /System/Library/PrivateFrameworks