You need to make sure FRED is loading the Inferno files. Whatever mod you select in the Launcher does not affect FRED.
To tell FRED to load Inferno: create a shortcut, right-click and go to "Properties". In the target box, after the filepath, put "-mod INFR1". (Note: the "INFR1" part should actually be whatever your Inferno folder is called). FRED will then load Inferno when you open via that shortcut.