Can't "Allow user to administer computer"

I know its simple but make sure the padlock icon is unlocked and that you lock it after the changes. Also you could see if theres an issue with the user or the system.

Create a test account and see if you have the ability to set it as admin.

This means try running disk utility; preferable booting from the install DVD and running the permissions repair and disk repair utilities. Then try again to change the user account settings.

Open the NetInfo Manager in the /Applications/Utilities/ folder, and without unlocking the padlock, look at the membership of the admin group; this group should normally contain an entry for root as well as one for each administrator account on the machine. This group's gid property is listed as 80 on my system. You can make a specific user an administrator by adding that user to the membership of this group; if the gid is something other than 80, click the padlock, type in your password, and change it to 80. Don't delete any users from the admin group, or alter any other accounts, groups, or properties.

Additionally, look in the list of users for the administrator account you previously tried to create; the NetInfo Manager shows the actual state of the Mac OS X system, while System Preferences conceals the more advanced information and controls.

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