The Frame.io FCPX Companion App can be used as a desktop app simply for uploading files and folders of files. After installing the app, simply drag files into the dotted line in the middle of the app.
If you are uploading a folder, the folder structure will not be maintained but the files will all be uploaded into the project.
Once you select them, you can select which project you want to upload these to. You will not be able to get to specific folders from the companion app.
If this is a new project you want to create, click the ... for more options.
You can also choose a default project that opens each time you're uploading. You can also view the project in your default browser or reload the projects displayed in the Companion App.