Unlocking Mac OSX with Admin ID Issues
Try the following :
Restart system - hold down Command-S to boot into Single User Mode(SUM) - this is the 'non-GUI' Command Line Interface and follow the solution that best fits your issue :
To Reset Admin Account
At the prompt, type the following, exactly as shown and press enter after each command listed. (case-sensitive)
/sbin/fsck -y [Enter]
/sbin/mount -uaw [Enter]
rm /var/db/.applesetupdone [Enter]
reboot [Enter]
This will restart the system as though the initial setup has not been run previously, follow the prompts to create a new 'Admin ID(User & Password) ** DO NOT USE THE PREVIOUS ID(or similar). When the system restarts after new ID has been created, the original Admin ID will still exist, but the new one will be the 'default Admin'. You 'may' be able to access all the data for the previous ID. If you can move it to another location, and you can then safely delete the old account. ** NOTE - It will not reset the Master Password
To Changing Admin Password
An alternate way is to change the Administrative password from SUM. Once you are booted into SUM, and are staring at the prompt, type in the following commands (in order), and hit enter after each one. Wait until the prompt re-appears before typing in the next command.
/sbin/fsck -y [Enter]
/sbin/mount -uaw [Enter]
sh /etc/rc
passwd username
Where "username" is the administrative account's username. You will be prompted to enter, and then confirm, a new password. As you type the password and confirmation, no characters will appear on the screen -- not even asteriks. This is fine, ignore it and keep typing.
More info and details can be found here :
http://www.macshadows.com/kb/index.php?title=Gain_Administrative_Control_of_Mac_OS_X