Hi,
The 3D Preview is usually already open when you start Terragen. It's the large viewport in the top right which shows the landscape and a small strip of sky in the default startup project. You can move around this view in a few different ways, such as holding the Alt key while dragging the mouse with various mouse buttons (see Help -> Mouse and Key Settings for more info) or using the navigation panel which can be opened by clicking on the navigation panel icon in the top right of the view.
There are a couple of ways that you can close this view, and so the 3D Preview toolbar button is there so that you can reopen it.
Matt
