Install Mountain Lion from InstallESD.dmg

You won't be able to use UniBeast directly with InstallESD.dmg:

Open dmg, and move "Install OS X Mountain Lion" into Applications folder:

Then right click on "Install OS X Mountain Lion", and select "Show Package Contents".
Go into Contents folder, and create a new folder named "SharedSupport":

Then drag InstallESD.dmg into SharedFolder:

You will then be able to proceed through UniBeast: Install OS X Mountain Lion on Any Supported Intel-based PC