Welcome to the forums, Bernd!
To subscript or superscript some text, select it and type Control-Shift-Command-plus (or in the Format menu, under Font, in the flyout menu Baseline, choose "Superscript"). For subscript, Control-Shift-Command-minus, or again use the Baseline menu.
To use a header or footer, you will need to put the text you want onto the slide Masters that you are using to appear on all the slides. To insert the slide number, select the Master you want, then in the Slide Inspector, click the Appearance tab, and check off the Slide Number option. You can now format the added number as you like.