For anything more than simple one-line formulas, use the Equation Editor. EE
is not installed by default, so check to see if you have it. If you see
Microsoft Equation 3.0 listed in Insert | Object | Create New, then EE is
installed. Click OK to that and a new equation object will be inserted.
If you don't see this listing, then you will need to rerun setup to install
EE. Go to Control Panel | Add/Remove Programs, select Microsoft Office, and
click the Change button. In the first page, select "Add or Remove Features."
On the next page, expand "Office Tools." Select Equation Editor and set it
to "Run from My Computer." Click the Update button, and insert the Office CD
when prompted.
If you will be using the Equation Editor often, you may want a toolbar
button to launch it. Open Tools | Customize and select the Insert category
on the Commands tab. In the Commands list, scroll down to Equation Editor,
select it, and drag it to a toolbar.