1) in at least one iso distribution (2008 R2 trial, maybe others) addprep.exe is in <drive>\support\adprep\adprep.exe and not <drive>\sources\adprep\
2) if adding an x64 installation new DC into an existing x86 installation, replace adprep.exe with adprep32.exe in all the instructions listed above where adprep.exe appears running on the x86 server, otherwise you'll get a nasty message about "adprep" not being suitable for your installation