Newb in need of help! WD media player not finding any files on my wd mybook

I have the 1.5tb mybook and when i plug it into the wd media player… nothing.  Just the welcoming screen with a no data stored or found at the top.  Now when i plug the mybook into the computer via an ethernet cable i can view the files from a network drive.  I thought the mybook was just a fancy networking capable external harddrive? am i wrong?  shouldnt it just work with a usb cable?  any and all help is appreciated, if you need more specifics i’ll rummage through some papers :)  thanks again.  -gary