A family of Microsoft word processing software products for creating web, email, and print documents.
If you are referring to the Equation command on the Insert tab, note that you must work in a Word 2007 document (*.docx) or template for it to work. You can't insert the new type of equation in a Word 97-2003 document (*.doc) or template.
However, you can insert the older type of equation in any document. On the Insert tab, click Object, and then locate "Microsoft Equation 3.0" in the list.