Under OSX it doesnt use the /etc/passwd for logins, it uses a database, the easiest method is to boot via cd and choose the reset password option, this changes root password. In future when you want to do adminning rather than using, login directly as root. Otherwise if you are uncomfortable with that give a user admin privileges, like you noticed it asks you for a passwd, what you are doing is a sudo operation. Hope that helps.
thanx,
bill