The All Files list may not show all of the files if you don't wait until the Building Songs Menu window goes away. If the list is blank, press the Reload button on the top right of the menu. (Version 1.5.8)
For older versions please see below.
If the list is blank, quit unrealBook completely, then try again.
Here's how to quit unrealBook completely:
Completely quit unrealBook by double clicking the home button which will open the task bar at the bottom of the screen. Press and hold on the unrealBook icon in the task bar (not in the main screen) until it wiggles. Then press the minus sign to kill the app. Now press the home screen and launch unrealBook the normal way.