To add the equation to your slide go to Insert menu, click Object and in the Object type list, click Microsoft Equation 3.0. If you can't find Microsoft Equation 3.0 you need to install it. Click on View > Toolbars > Customize from the PowerPoint main menu and go to Commands tab. In the Categories list, click Insert and click on Equation Editor from the Commands box. Drag the Equation Editor to a gray area where are the others toolbars and click Close.
After you type the equation click Exit and Return to from the Equation Editor File menu.