Excel is even worse, as neither command appears in the list. In PowerPoint’s Commands tab, you’ll only find Superscript, not Subscript. You’d think it’d be just as simple in PowerPoint and Excel, but that’s not the case.
Drag each one to a spot on a menu, and you’re done. Once the customization dialog appears, click on the Commands tab, then scroll down to Subscript and Superscript in the list. In Word 2008, it’s in View -> Customize Toolbars and Menus.
In Word 2004, you’ll find that menu in View -> Toolbars -> Customize Toolbars/Menus.
In Word, you can use the toolbar customization menu to add the super/subscript buttons to a toolbar. At least, you should be able to add those buttons to toolbars…however, Microsoft has made it somewhat maddeningly difficult to do so, as the process varies depending on which application you’re using. If you work with a lot of superscripts and subscripts in Office (both the 2004 or 2008 versions of the suite), you can make your job easier by adding the superscript and subscript buttons to the toolbar in Word, Excel, and PowerPoint.