In the Drawing menu toolbar, click the icon of a T in a square to create a text box. Once the text box is created, click the Save and Close button in the top-right to add it to the document.

In Word 2003, choose Text Box from the Drawing toolbar (View, Toolbars, Drawing). Use the mouse to drag and drop a text box of the approximate size in the margin.

Whether you're putting together some kind of mock-up document or trying to learn Word's formatting features, filler text can come in mighty handy.