insert => object => Microsoft Equation 3.0 => choose the equation format you need to use.
PS: i don't know if you can find the Equation successfully. Somehow I didn't have it in my word initially. If you don't have it, find a word file that has the equation format, and then double click the equation, you will be asked to use the initial MS Office installation CD to install the equation as a plug-in. After you finish installing, enjoy it.