A publisher or academic department may ask you to use a style that is not listed in the Select another style list within Word's Style menu. Where a file for the style you require is available from another source, you can can download it.
You should then be able to select your style by going back to Word and using the drop-down Style menu to Select another style. You should now be able to see your style in the list and to select it.
Please note, if you are using EndNote Web there is not an option to download citation styles.