You don’t need to install anything to write or edit math equations in Word 2007 and later. Just choose Insert > Equation.
Equation Editor (Microsoft Equation 3.0) was included in previous versions of Word and is still available in case you need to edit an equation written in a version of Word older than 2007. Double-click the equation you want to edit, and the equation editor will open automatically.
Important: This feature isn’t available in Office on a Windows RT PC. Want to see what version of Office you're using?