Add this param to your flash movie parameters

name="wmode" value="transparent"

The easiest way is simply to paste the html straight into a text box using the Source button. Typically your code may look something like this;

+++++++++++++++++++++ CODE

<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" 
codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0" width="558" height="198">
                  	 <param name="allowScriptAccess" value="sameDomain" />
			 <PARAM NAME="BASE" VALUE=".">	
  <param name="movie" value="/Portals/0/yourfile_as1.swf">
<param name="quality" value="high"><param name="wmode" value="transparent" />
                      <embed src="/Portals/0/slideshow_yourfile.swf" BASE="." quality="high"
pluginspage="http://www.macromedia.com/go/getflashplayer" type="application/x-shockwave-flash" 
wmode="transparent" width="500" height="200">
</embed></object>

++++++++++++++++++++++++++ ENDCODE

Change the various parameters to suit your needs

Alternatively there are modules out there that do the job for you. www.inventura.com is a good one.