The first method for transferring .fadein documents to and from Fade In Mobile is via a supported cloud service, such as Dropbox, Google Drive, OneDrive, or iCloud (on iOS).
You'll need to select the relevant service from the menu, authorize Fade In Mobile to access it, then browse to the document you'd like to import and tap the filename.
To export a file, you can long-press the name in Fade In Mobile's document list (or tap the three dots to the right).
Fade In Mobile will give an indication of the status of a document, whether the Fade In Mobile version is newer or the source version is newer (i.e., if it's been modified since it was imported or last exported).
You can also access files in the user storage of your device using "Device Storage" as the source, the same way you would use a cloud service.
You can also access files via the "Files" interface of your device by selecting iCloud as the source. (iCloud and Files share the same interface.)