Click on the Start button > type %appdata% into the text field > ok. You should now see a lot of folders - find the one named LimeWire ... that is your preferences folder.
The gnutella.net file is situated in that folder. The folder itself is not called preferences, simply LimeWire.
The appdata\roaming is where preferences for most of your programs will be stored. Doubtful any of them will have the word preferences suffixed at the end of their title.
ie:
C:\Documents and Settings\*yourusername*\AppData\Roaming\Limewire