Very odd indeed. It you want to follow Carolyn's advice you need to be in the correct Library folder. The path you have given is for the general system Library, which is at the root level of the drive. You want your home ~/Library folder (the tilde matters). There are several ways to open it, now that Apple's Infinite wisdom has declared it should be hidden.
1. Open your user folder, bring up the Show View Options either by Command-j or selecting that from the View menu. At the very bottom of the list of options there is a radio button to Show the Library Folder. Check it. The ~/Library folder immediately appears. Open it, then open Preferences and delete the com.Apple.dock.plist file.
2. Open the Go window and press and hold the Option key and ~/Library folder magically appears in the list, click on it and remove the file.
3. Copy and paste the path Carolyn gave you in the Go to Folder option of the Go menu, find and delete the file. Restart and see if things are correct.
If that doesn't do it, launch Disk Utility and run repair permissions. It won't hurt and sometimes it actually helps. This peculiar problem may be one of those times.
Francine