To do Alt + 8 on Mac in PowerPoint, download and install Accelerator Keys, then press Option, followed by 8.
Command: Quick Access Position 8 (user-customizable)
This is how to do Alt + 8 on Mac in PowerPoint with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with PowerPoint.
Alt + 8 activates the eighth button in your Quick Access Toolbar. This position may not have a default command assigned, but you can customize it to any command you frequently use. The Quick Access Toolbar appears at the top-left of PowerPoint, making these shortcuts some of the fastest ways to access your most-used features.