If you can't find the file, simply make it using a text editor and place it into the Fallout 4 folder. Once you find the folder, look for the file 'Fallout4Custom.ini' and then open it. The first step is to find your Fallout 4 folder, usually through the path User\Documents\My Games\Fallout4. Before anything, you first have to enable mods for your game.
