Actually, the plist file isn’t in the /Library/Preferences folder, it’s in the Library folder in your Home folder. Mac OS X has a few different Library folders, which can make things confusing.
Please check in Macintosh HD/Users/<Your Username>/Library/Preferences for a file named “com.skjm.icamsource.plist”. Make sure that the Username is that same as the account used to launch iCamSource.