There are several possibilities here:
1. The Standard toolbar is not displayed.
2. The Standard toolbar is displayed, but it is sharing a row with the
Formatting toolbar, and only the most recently used buttons are showing.
3. The Standard toolbar is displayed on its own row, but the Zoom box is
missing.
For (1), right-click on a toolbar and choose Standard. For (2), click the
arrow at the end of either toolbar and choose "Show Buttons on Two Rows."
For (3) see
http://word.mvps.org/FAQs/Customization/AsgnCmdOrMacroToToolbar.htm or click
the arrow at the end of the Standard toolbar, choose "Add or Remove
Buttons," choose Standard, and click on "Zoom:"