By default the Formatting toolbar has buttons for Left, Center, Right, and
Justified alignment. If you are missing some buttons it's because you've got
adaptive menus enabled or are showing the Standard and Formatting toolbars
on a single row, or it could be that you've somehow lost that button. See
http://word.mvps.org/FAQs/Customization/AsgnCmdOrMacroToToolbar.htm and also
http://word.mvps.org/FAQs/Customization/RestoreMenuCmd.htm (applies to
menus, but all three methods can be adapted for toolbars).