The first thing to check is if _other_ USB storage devices are okay -- that is, if Ubuntu automatically mounts them after you plug them in. If not, then the issue is generic for all USB storage. If it does mount those, but not your Kobo, that's a different issue. Report back here when done with this test.
Oh, and: You should probably change the title of your thread/post so that it has the word Linux in it. (Maybe a moderator could do that for us?)