i wanted to test the x64 version for a bit, but didn't see the option on steam (i guess it's their issue)
so i executed the DayZ_x64.exe, got the error "Unable to locate a running instance of Steam" (with steam running). Game was starting, but taskmanager showed it as 32-bit.
the error message showed nothing in the taskmanager only "DayZ", so i guess it was the x64 version.
i then checked logfiles and the DayZ_x64.rpt showed:

C:\Program Files (x86)\Steam\steamapps\common\DayZ\DayZ_x64.exe

"C:\Program Files (x86)\Steam\steamapps\common\DayZ\DayZ_x64.exe"

Exe timestamp: 2016/12/06 18:44:33
Current time: 2016/12/06 21:26:21

Version 0.61.136505

ErrorMessage====>>>>>>BEG

ErrorMessage====>>>>>>END

ErrorMessage: Unable to locate a running instance of Steam

sneakydude told me on the forums to swich back to stable and then directly back to to exp, so i don't have to dl stuff.
steam now showed me x64 option.

the problem with startign from the folder remained and is there for both versions (although the 32bit version starts in both cases).