Welcome to Wiihacks, I'm sure a proper welcome from a mod will be along soon. In the meantime...
I'm pretty sure the only usb loader that reads FAT formatted drives is CFG-Loader. You'll need to reformat WBFS to get Wiiflow to read your drive. Take a look at the 3rd link in my sig.
Rev15 is a wash at the moment. That is going to be a source of trouble once you get your usb drive working. You'll want to uninstall it and get rev14 on there. Also check out whichever of the top two links in my sig contains your firmware version to make sure you aren't missing anything important in your softmod.
Finally, check out USB Devices Compatibility List - WikiTemp to make sure there are no issues with your usb drive.