this looks, like Aperture is unable to write the preferences file or your presets.
See the Aperture 3: Troubleshooting Basics on how to remove the com.apple.aperture.plist file from the Preferences folder and how to remove the command presets.
You did not say, what your Aperture version and your MacOS X version are. In MacOS X 10.7.x your user library is hidden. You open it best from the Finder's "Go" menu, from the main menu bar:
Go > Go to folder then hold down the alt/option key until the library appears in the drop down menu. Select it to open.
Also, with the new process model in Lion and Mt. Lion I'd log off and on again, before removing preferences files. Quitting an application alone will not always suffice to terminate the processes.
Thanks for the response. This didn't solve the problem. I'm moving from library to library often, and that appears to be resetting it, even though the preferences for each are set (and reset) identically. Does that make sense?
Preferences continue to be erased on a consistent basis : (