Why can't I get my slide numbers to show?

Edited November 18, 2015, to add this information:

Microsoft has started using powerpoint.uservoice.com for feature and bug fix requests from the PowerPoint community. Have an issue that's not getting the attention you want? Post it on uservoice and then talk to your friends about throwing some votes at it! I added the slide number/header-footer issue; now let's vote it up!

http://powerpoint.uservoice.com/forums/288949-powerpoint-for-windows-desktop-application/suggestions/10609608-make-it-easier-to-get-my-slide-numbers-and-date-a

For now, here's what we're stuck with:

You need two things for a slide number -- a placeholder, and to populate the placeholder.

PowerPoint messes up the whole structure -- or seems to -- when you open 2003 and prior files in 2007 and 2010 (and 2013 and 2016). They really mucked stuff up! It's partly because the whole masters paradigm is different now, but they still blew it. (Trust me, I've complained mightily to the PPT developers about it. I will continue to do so.)

Here's the most straightforward way to fix them that I know.

Go to the slide master (the big thumbnail), select any footer / slide number boxes and delete them. Now look at the slide layouts (little thumbnails). Delete any footers / date / slide number boxes on each layout.

You shouldn't have to do this part, I know. But because it's all messed up, this is the most surefire way I know to correct the issues.

Now go back to the Slide Master (big thumbnail), click Master Layout on the Ribbon and tick the boxes for date, slide number, and footer. Format and position the placeholders as desired.

Now select each slide layout and tick the Footers box on the Ribbon.

Then close Master View.

Now go to Insert | Header and Footer (or Insert | Slide Number is fine, too) and tick the box there for slide number. The slide number placeholder you created should be populated with the slide number.

The key here is having the placeholder on all the slide layouts. When you add the placeholders to the slide master (big thumbnail), the associated slide layouts (little thumbnail) should inherit them. But part of the problem is that any "leftover" layouts from PPT 2003 don't always inherit the footer (and date and slide number) placeholders properly.

And there's other weird stuff that goes on.

The other thing is, if you paste or insert other existing slides, you may need to do this all over again. That's because you may end up with additional slide masters or layouts after pasting in slides. So I recommend you do it after you've added all the slides you're going to add.

Hopefully this will get you going.