Go to the Insert Ribbon and click on the Header & Footer icon in the Text Group.
Choose the options you would like, including typing text in the Footer box, and then click either the Apply or Apply to All button.
If you want to change the position of the header or footer, move the placeholder text box on the Master Slide (see the Master Slide tab for more information).
Notice that by default, you are editing the header and footer for the slide that will display during your presentation. Click on the Notes and Handouts tab to edit the header on those instead.