Ubuntu mounts flash drives as you insert them. You can access them from the
Places menu under removable drives or in the notification area under the usb
icon. Sometimes they will not mount properly. This often has to do with it
not being removed properly on the previous use. I have found it tricky to
get it to work in that session, but once it is mounted and removed properly
in another OS or after a re-boot it will often work fine.You can also try to
format it in Partition Editor if there is nothing on it. Occasionally a usb
stick just won't work any more in any OS.