fsv's user interface relies heavily on the mouse. Mouse functionality differs only slightly between the left and right panes of the main program window.
In the directory tree / file list area (left pane):
The left button selects a node entry.
The middle button doesn't do anything.
The right button brings up a context-sensitive menu for a node entry.
In the graphical viewport area (right pane):
The left button selects a node, and sends the camera toward it.
The middle button dollies the camera forward or backward. (click & drag)
The right button brings up a context-sensitive menu for a node.