Use Insert > Object > Formula to create a Math object in a slide. When editing a formula, the main menu changes into the Math main menu.
Care should be taken about the font sizes used in order to make them comparable to the font size used in the rest of the slide. To change the font attributes of the Math object, select Format > Font Size from the main menu bar. To change the font type, select Format > Fonts from the main menu bar.
For additional information on how to create formulas, refer to Chapter 9 (Getting Started with Math) in the Getting Started guide, or Chapter 16 (Math Objects) in the Writer Guide.
Note that unlike in Writer, a formula in Impress is treated as an object, therefore it will not be automatically aligned with the rest of the text. The formula can be however moved around (but not resized) as any other object.