Click on one of the sources areas in the Source radio box
 to specify where the files listed in the selection list box should
 come from.

 You have the option of picking different directories to list the files
 from or of indicating that files should be listed from the CMS library.
 You can go from directory to directory to list selection files from, but
 if you then choose the CMS library, the Input (left-hand) selection list
 will be cleared; and vice-versa.