Sadly, these USB to legacy devices don't always work as advertised. Scanners, especially film scanners, seem to work only when directly connected to a SCSI interface. If the USB device is cheap it may be worth a spin, though it's probably intended for PC not Mac.
If it's any comfort this is a common problem for Mac owners due to Apple's typically arbitrary decision to drop SCSI after having used it for years -- thus the several SCSI flatbed and film scanners recovered from dumpsters and flea markets I now use with my PCs.
You might look at Plustek's rather good value USB scanners as an alternative.