Dear Bob, Paul, and all,
Thank you for your replies.

)) Oops! Looks like you missed the step to "Build Devices" 
)) in the Installer.
Yes, I actually forgot "Build Devices" step.
I could finally login as a root onto my Mac with NetBSD/mac68k.
after choosing the "Build Devices" menu item while in the 
Installer.

And I found the same answer on the following URL, too.

  http://www.jp.netbsd.org/Ports/mac68k/faq/

Thanks again for your help. Bye for now.

--
   Yamazaki, Masayoshi / Osaka, JAPAN
  E-Mail:yamazaki@kwanghwa.to [Japanese]
        masayoshi@hanmail.net [Korean]
    http://member.nifty.ne.jp/BONZ/