When you turned it on you may have noticed that the Media Directory defaults to "/mnt". I tried a few changes, but that seems to be the only value that works. On your USB drive, create a directory named "mnt" and place your media files within.
That worked for me, but now that I took my drive off and added in a lot more files and re-connected (and re-booted router) I can't see my files. So YMMV on the above.
--UPDATED: Looks like since it was a lot of files, just took the server a while to index them. I can see all of them now. So yes, place them within a "mnt" folder and you should be golden.