Plug in your usb device.
As root type this:
Now you should see your device listed. Make a note about the devicename/mountpoint.
Next make a mountpoint for your device:
You probably want to access your usb device as a regular user, so change permission by typing this:
chgrp cdrom /media/usb0
chmod a+rw /media/usb0
Next open up your fstab using an editor:
Add this line to the end of the file:
/dev/sda1 /media/usb0 vfat rw,user,users,noauto,noatime,noauto 1 2
Now remember to change the 'sda1' into the info you got earlier when you typed the ls -l command.