I suspect that by the “Fuze application” the OP means the little Fuze icon that comes up in Windows Explorer in MTP mode.
But for testing purposes, go to Settings/System Settings/ USB Mode and put it in MSC, the simpler mode. In MSC, you should see SANSA FUZE and Removable Disc, which means the card slot.
If you see Removable Disc, what happens when you click on it? Can you see files on it?
Also: is the MicroSD card formatted to FAT32?
