The prefs folder isn't within Program Files
You'll need to make sure that your computer is set to show hidden files & folders because the prefs folder is within an Application Data folder, which is hidden by default.

Which OS are you using? There's info for both XP & Vista in the link that LOTR gave in post #2... if you're using Vista then you need to click on the 'prefs folder in Vista link' as well: LimeWire Pref Folder in Vista

If you're using XP then there are very basic instructions within this thread (click on link): If LW Will Not Connect


Are you using LW 5.0 or LW 5.1? Open LW, click on Help > About LimeWire & you'll see the version number.