You could use the keyboard shortcut CTRL+j, or Format>Paragraph>Indents and
Spacing>General>Alignment - Justified, or add the toolbar icon to your
toolbar by right clicking the toolbar and selecting customize. You will
find the icon in the format category. Just drag it to your toolbar.
Just a thought - perhaps someone has removed this command from the interface
to train you not to justify text? For most types of document left aligned
text is much more suitable, and Word's justification really isn't very
clever <g>.