You need to mount the usb drive to use it. First plug it in, then run this command in a terminal
This will tell you which device node the usb was assigned to (like /dev/sdc or /dev/sdb etc. ). After finding that out create a new directory in /mnt (call it "usb" or whatever). Then use this command as root.
mount -t vfat /device/node/for/usb /mnt/usb
Don't forget to substitute /device/node/for/usb with the actual device node you found out with dmesg. After doing this simply navigate to /mnt/usb and you can access your files, once you are done run this command as root
Then you can safely remove your usb.