Let see the picture below to understand the Bexpand main window. 

Main window.

The instrument list define all the instrument Bexpand use. By selecting an instrument or a sample within an instrument, you can play it with external or internal Midi keyboard/sequencer. When you launch the software, you have currently one instrument (empty). You can load a sample in this instrument with the File menu. To add instrument or sample in an instrument (sub sample), use the Instrument menu. And to configure all the samples in an instrument use the Parameter Item of the instrument menu.

This is the general information about this window, documentation is more complete in the full version of Bexpand.