To collapse a node with the keyboard, first make sure that the tree pane has focus.
'Having focus' for the tree pane means that any key that you press on your keyboard will be sent to the tree pane.
To make sure that the tree pane has focus, please first click on it with the mouse.
(1) Use the up/down cursor-arrow keys on your keyboard to select the node.
(2) After this, press the left cursor-arrow key on your keyboard
- or -
Press the plus (-) key on your keyboard's numeric keypad.