If the "Microsoft Equation 3.0" option doesn't appear in the "Insert/Object…" dialog (in Word, PowerPoint, Excel, etc.), then it was not selected for installation when Office was installed.

To correct this, you will need to go into the Windows Control Panel, and run the "Add or Remove Programs" option. Select Microsoft Office from the list of installed software, and click the "Change" button. This will launch the Office Setup program (depending on how Office was first installed, you will probably need to insert the Office installation CD or DVD before proceeding).

In Office Setup, select the "Add or Remove Features" option, and press the "Next" button. In the next panel, check the "Use advanced customization" checkbox, and click the "Next" button. You will then see a hierarchical list of Office applications and tools. Expand the "Office Tools" item, and you will see the "Equation Editor" option. Click on that item, and select "Run from My Computer" from the pop-up menu that appears. Then click the "Update" button at the bottom of the Office Setup window.

Following the above procedure will install Microsoft Equation Editor, making it available for use in Office applications (and all others which allow the user to insert embedded objects)