footer won't show up on slides

If you want to use the footer placeholders,
switch to Normal mode, select Insert - Slide Number
from the menu bar, put a check on which placeholders
you want to add to your slide(s), and select either
Apply to All or just Apply for the current slide.
This will apply the default footer placeholders
to your presentation.

You can modify the location and style of your footers
in the slide master. You can also add your own
placeholders, text boxes and so on to the
slide master, as you've probably already found out.

Hope this helps.

Jeff