Ah. You did not say that you had already tried that.
When the changes do not stick, this may be because permissions make the property list file unavailable to be re-written with the new information.
There are two Library folders:
The first contains the system-wide preferences set by the original Administrator. Its permissions can be reset by using Disk Utility ( Repair Permissions ). [it is always prudent to perform a Disk Utility ( Repair Disk ) first. If not available, a ( Verify Disk ) will suffice as long as no repairs are needed.]
The second is your personal Library, which is located at /Users/your_user_name/Library but is often invisible by default. You can use the Finder's GO menu, choose Go To Folder, and enter: ~/Library/Preferences
The file that appears to hold these preferences is :
Opening that file may damage it. But you can GetInfo on the file and be sure that under Sharing&Permissions, Owner is your_user_name, and that Group:staff has permissions Read&Write