You can either select the resource file from the "Working Directories" tree and drag it into the desired session. Or you can add a resource via the context menu (right-click) "Add/WrittenResource" or "Add/MediaFile". However if you add via the context menu rather than dragging you will then need to manually edit the location and type of the resource.

Only some file types are archivable and this is indicated by the icons in the "Working Directories". If a file is not archivable then you will not be able to drag it into a session.