To use puzzle shapes, you need to go to the 'Settings' window and then to the 'Puzzle Shape' tab. To activate shapes for your puzzles, check the box for 'Use shapes for puzzles'.
You can then select the set you would like to use shapes from and then select the shape that you would like to use for the puzzle. Press the 'Done' button.
You can also switch the shape from the main window. The normal width and height settings have been replaced with the shape set and selected shape settings. This makes it much easier to switch shapes.