How do you set the MySQL password on Mac OS X?
Are you getting an access denied error or something else?  mysqladmin should be printing a second line with more details, too. Have you looked over the installation notes for 5.5.25 (the current GA) at http://dev.mysql.com/doc/refman/5.5/en/osx-installation-notes.html ?

Sometimes the client don't look for the mysql socket file in the same place as it was written, or something deletes it if it's written in /tmp or another similar location.  If so then connecting over TCP might work (add -h 127.1 to the command line options).

If you need to reset the password, you can stop mysql and start it (temporarily) with an option to just allow any access without authentication.  I find the easiest way to get the current options it's running with is ps axwwwwwww | grep mysql.