This is pretty much a html problem and depends a lot how your site is structured.
For absolute centering (horizontal + vertical) on an empty html page use sth. like this in the css of your Html-wrapper
/* CSS Document */
width: [insert stage width of your flash file];
height: [insert stage heightof your flash file];
margin-left: [insert half of the negative stage width of your flash file];
margin-top: [insert half of the negative stage height of your flash file];