Adding footer

I have my slide set up but would like to add a footer with the hymn number. How would I do this?

The footers for Song cues are pulled from the song's details. You can access this by opening the song as if to edit the slides, then clicking on the title of the song near the top of the Presenter screen. There isn't a box dedicated to hymn number, but you can add it before/after the song title, author name, or Copyrights line and it will show on your footer.

I don't have anything showing up on the bottom of the slide. How do I change it to show the license number and title?

Settings > Footer
Secondly, make sure song details are filled out as mentioned in my previous response. If this is filled out, the footer should be visible. You won't see it on the Presenter computer, but you will see it in the bottom left Preview screen, or on your presentation screen.
Note: this only works on Song cues, not Slides cues.