Basic build mode shows when you are in maximize mode (it's not available when you are in the pageview). To get into basic build mode, you can click on the toolbar in the top right hand corner of the screen. You can select, create and place a prefab from here. There are several short basic build tutorial videos to view; they are also available in the tool under each question mark. Or you can follow this tutorial on how to customize a prefab block house to give you a general idea.
Basic Mesh Building
Cloud Party Video Tutorials