To edit streamfiles, you can use the IFS Browser. The default folder that will be displayed is the home directory which is defined in the ILEditor 2 settings.
When you click on a directory, it will load all directories and files below it. The file extension correlates to which syntax highlighting is used. When you click on a streamfile, it should open up in a tab where the code can then be edited.
You are able to right-click on source physical-files and members which will show you the related context menus.
You can save with Ctrl + S / Command + S, plus all regular editing shortcuts can be used (like undo, redo, etc).