How does one use the accelerator key for a command button in a PowerPoint slide?

It's because the accelerator feature has been turned off in recent versions of Microsoft Office.

Problems were reported when people would put a text box that acts as an input field onto a userform. On the same userform they put a button or control with an accelerator key. When the form is run, while you're typing in the input form and happen to type the letter of the accelerator key, the accelerator action would execute and that made it impossible to fill in the forms.

To resolve the conflict, Microsoft disabled accelerator keys, although the VB editor still lets you make them. A better solution would be for Microsoft to detect whether the cursor is in a text box and only disable the accelerator key when that is the case.

If you want Microsoft to fully resolve the conflict, send them a note.

For windows:

http://mymfe.microsoft.com/Office/Feedback.aspx?fo...

For Mac

In an Office application choose Help > Send Feedback