Each file opens in its own window. Use File › Open… to browse, or File › New to start a blank one.
Files saved by version 1.x can't be opened yet. Files from version 2.0 and later open;
those saved before version 3.0 (2016) are untested, so keep a backup before saving over one.
Data tips
Tick what the tooltip shows when the pointer is on a person or a line. The window a double-click opens shows everything, but IDs and picture details (file, size, date) only when ticked.