There are two different ways you can pan the display. The simplest is to select the pan tool and then click-and-drag in the treemap.
Better than that is simply to click-and-drag with your middle mouse button, if you have one; or, if you have a wheel, press down on the wheel in the treemap and then move the mouse with the wheel pressed down. This avoids having to switch tools.
And, of course, you can also use the scrollbars at the edge of the treemap.