The .img file is a disk image that is written to the USB memstick using dd, win32disk imager, rufus, etc. The filesystem format is UFS which few other operating systems can read, aside from FreeBSD. The fact that you can't see any files on there does not mean there is a problem.
Try to boot from the USB memstick. If the hardware supports booting from USB and FreeBSD will run on that hardware, then it should work.
There was at least one other person who managed to get it to work on similar hardware, but that might be a different model: https://forum.pfsense.org/index.php?topic=116910.0