How to center Drop-Down Box

[remp]

Hello. im using a script from DD (http://www.dynamicdrive.com/dynamicindex17/amazondrop.htm) which allows me to set the position of the drop-down box. now, the problem is that when somebody has a bigger monitor (screen), they see the box outside of the website margin, how can i change this so it can alway be centered?
i really appreciate any help.

[Medyman]

http://d-graff.de/fricca/center.html

[remp]

i need something like that but on that website theres no information on how to do it.