To quit the program, close all open panels and regain control of your avatar and then hit ESC and then select YES on the confirmation dialog to exit.
Or alternatively, hit TAB or click the APPS button, and select SETTINGS from the menu wheel.
On the settings panel that appears, you’ll see a blue button that says QUIT [R&D.Construct] in the bottom left corner — click that, then click YES on the confirmation dialog that appears.