Well just use logic to follow the guide as it applies to you:
run regedit.exe go to the relevant key:
32bit Windows:
HKEY_LOCAL_MACHINE\SOFTWARE\Bohemia Interactive Studio\
64bit Windows
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Bohemia Interactive Studio\
Delete all entries relating to Operation Arrowhead, then launch Steam
ensuring you launch it as admin
http://img717.imageshack.us/img717/5892/steamadmin2.jpg , once Steam is
launched go to the library, install Operation Arrowhead again, wait
until it's finished auto updating, once that's finished exist Steam, go
back to regedit.exe and check that the registry keys for Operation
Arrowhead have now been created, if they have then you should be able to
play OA with your own CD K*y.