Just plug a USB pen in the box.
In the Status page it will appear in the Disks section, notice its device (Dev.) name, usually sdb, sdd,....
In Disks->Filesystem you will see one or more items (filesystems) under the same device name (Dev.), depending on whether or not the pen is partitioned (ussualy it is not). It can be sdb (not partitioned), sdb1 (first partition on sdb device), sdb2, or sdc, sdc1...
The USB pen files will show up under /mnt/sdb, or /mnt/sdb1, or /mnt/sdc...
If when you plug the pen under windows the secret file is at g:\mysecret and under Alt-F the pen is named sdc, your file will be /mnt/sdc/mysecret
Notice that there are some issues with encripted filesystems, see http://sourceforge.net/p/alt-f/tickets/?source=navbar, and
its slow. If you are using disks partitioned under D-Link firmware I advise you to start using LUKS at the sda4 or sdb4 disks filesystems, as they are small enough to be usable.