Note: Microsoft Office applications (Word, Excel, etc) do not play by many of the standard rules of Mac OS X. You can modify Office toolbars but the process is different. See Office help files for assistance.
When the Customize Toolbar sheet appears, you will see a selection of available buttons you can add. You can drag any of the buttons on the sheet directly to the Toolbar. You can also move existing buttons right or left to change their position.
If there is a button on the Toolbar that you don't want, drag it off the Toolbar and it will disappear with a "poof".
In addition to buttons, there is a Separator that adds a vertical line and a Space to add space between buttons. These can help you organize like operations.
If you make a mess of things or just want to go back to the default Toolbar, drag the Default Set to the Toolbar. This will remove all custom buttons and put things back the way they were originally.