It has nothing to do with OpenBSD, as I said.. Macs use EFI + GPT partitioning, from what I can find.. you need to use a 3rd party boot loader.
http://refit.sourceforge.net/
http://en.wikipedia.org/wiki/Boot_Camp_%28software%29
The closest guide I can find for you is for NetBSD.
http://wiki.netbsd.se/How_to_install...ook_w/core2duo
Your best bet is to send an email to the mailing lists, there have been successful installation reports on the mailing lists.. someone may know the procedure.