Use the PowerPoint menu to go back

By default, when you are viewing a PowerPoint presentation pressing the right mouse button shows the PowerPoint presentation menu.

If you'd prefer that, instead, it moved back one slide, then you can configure it to do this. Choose Tools, Options and click the View tab. Clear the Show menu on right mouse click checkbox and click Ok.

Now, the left mouse takes you forward and the right takes you back. If you need to see the menu at any time, pressing Shift + F10 will display it.