[Left mouse button]: Use the left mouse button to rotate the object freely in space.
[Shift]+[Left mouse button]: Locks rotation to the current viewing plane.
[Scroller]: Use the mouse scroller to zoom in and out.
[Right mouse button]: Use the right mouse button to translate the object freely in space.

Notice the box in the bottom right corner of the screen shown. This defines the coordinate system and moves when the object moves. Use this to help determine the “up direction” and maintain your bearings when working with a model. To snap to a particular view, click on any face of the cube shown in the coordinate system.

Watch this Intact.design beginners' tutorial video at 2:38 to learn more about navigation controls.