To add files to the AudioCommander file list, click the "Add Files" link on the main interface or select "Add" from the software's Edit menu.  You can also add all the audio files contained in a given folder by using the "Add Folder" option.  When using the Add Folder command, you can optionally include files contained in subfolders of the root folder selected as well by enabling the "Include Subfolders with Added Folder" option.