So you have to point vbox6 to /recall_fs/debian-13.1.0/vm.
Installing the debian-13.1.0 pkg and downloading the .iso into the recall_fs went fine here. However, I noticed that you cannot use the … button with the recall_fs.
From the vbox6 dialog one has to navigate via
used file system → recall → debian-13.1.0 → vm
to directory containing the assets.