To elaborate:

D|S since version 2.1 has two ways of viewing content:

1. Folder views, which show the folder structure as it's laid out on the hard drive, and
2. Category views, which use a database.

Category view lets you rearrange your content in whatever categories you want without changing the folder structure on the disk.  However, to use it you have to do a Search for Content in order to populate the database.  This puts everything it finds in the Unassigned category, from which you can move it around wherever you like.

Category view is the default, so you need click the triangle in the upper right corner of the content tab and either:

1. Change to a Folder view, or
2. Search for Content.