the current pipe segment is the bottom one in the stack (on the left of the screen).
To drop it, click when the desired box on the playing grid is selected.
Once a piece is dropped the stack scrolls automatically. You can scroll the stack without putting any piece on the grid by clicking on the “Arrow” button or by pressing the assigned action key (default, Shift).
You can destroy wrong placed pipe segments or reveal bonuses (those marked with a “?”), using the “Bomb” button, either by clicking it or by pressing the assigned action key (default, Option). Both actions are allowed only the number of times specified by the number shown in the button.