well for starters I'm not sure that is what you wanted to do.... Secondly you might have tried searching the forum as this has been answered in detail a number of times..

So IF what you want is the background tansparent, click on the File menu, then Export Settings and on the HTML tab is a check box for transparent background. This assumes you will use the HTML code provided by Koolmoves, if not just understand that all SWF's are transparent by nature and the background color is set via the HTML unless you create a background in the flash itself. So you'll have to find out how to set transparent background in html for what ever method you are using to create the HTML portion of your webpage.

No need to be snippy, you might note that I have more than a few posts here and offer my help because I like to and for no other reason nor do I get paid to do this.