It is mainly located right beside the Microsoft Office Button. By default it shows just three commands; Save, Undo and Redo.
See the image:
When you click on the drop-down arrow at the right end of this Quick Access Toolbar, it will offer several more commands. The desired command out of these displayed set of commands can then be added to the Quick Access Toolbar with a left click on it.
See the image:
You can also similarly add the Office Button and the Ribbon commands. Just right click on the command and select the “Add to Quick Access Toolbar” option present there. See the following image, the command ‘New Slide’ is now added to Quick Access Toolbar.