Are you sure you are viewing the user library and not the system library?
The User Library (username/Library) is hidden by default in Mac OS X (10.7.5).
To open it in a Finder window use the "Go" menu from the Finder's main menu bar:
- Quit Aperture, if it is running. Log off and on again.
- Open the user library by using the Finder's "Go > Go to Folder" menu and hold down the options-key, until "Library" appears in the drop down menu. Select it.
- In the widow that will open, scroll down to "Preferences"
- From the "Preferences" folder remove "com.apple.Aperture.plist".
Then try to launch Aperture again.
I am pretty sure, that the com.apple.Aperture.plist file must be there. If the file is not there, Aperture will recreate it when launching.
In the rare case, that Aperture cannot create this file, because the user library is not writable,
Then Aperture will write to $TMPDIR instead.
Check this temporary directory - you can open it from the Terminal: Launch the Terminal.app from Applications > Utilities and and enter the following line:
Hit return. This will open a Finder window. Look for a folder "com.apple.Aperture"
You can delete its contents.