Left click on an object to select it, denoted by a blue highlighted box around it. Left click and drag in an empty area to draw a select box, allowing you to select multiple objects.
Left click and drag any selected Stuff to move all selected Stuff around your level. When your cursor is in the corner of a selected Stuff, it will render , left click and drag to rotate all selected Stuff.
Left click in any empty area to deselect objects.
Right click on an object to open its Context Menu. Right click and drag to pan your view.
Use your mouse wheel to zoom your view in and out. Zooming via mouse wheel will use your cursor as a focus point.