Check the Key Preferences menu item in the Game menu to see which keys do what.
When you think the putter is positioned correctly, use the swing key to hit the ball. Hold it down to draw the putter back. When you let go of the swing key the putter moves forward to hit the ball. The further you draw the putter back the harder you hit the ball. If the putter reaches its maximum backswing position it automatically begins its stroke.
