Obstructions such as buildings can be added by using the Add new object tab below viewports.

In the tab, you can select:

After adding the obstruction, you can edit it in lower viewpoint by clicking on it. Depending on selected mode, you can either move, rotate or scale the obstruction. You can select the mode using buttons at the bottom of lower viewpoint.

Upper viewpoint allows you to look around in 3D, to better see the obstructions. Objects tab lists all obstructions currently in use and gives you the ability to delete the obstruction.

After you are done placing the obstructions, you can simply click the Apply button on the left side of the page. This will save the obstructions and other settings, such as your position. Your new obstructions should be applied and you can continue to the other pages.