To use flash on Joomla you need to follow the following steps:

1. Login to back-end of joomla site.
2. Select Component -> Banners.
3. You will Banner Manager page. Click on new button on the top left side.
4. You will see a form to add banners. Enter the details given below: Name: Enter the name of your banner Category: Select the category for the banner. Type: Select the custom as the type of banner. Custom Code: Add the following code:

 
 <div style="width:150px;height:100px;">
 <object type="application/x-shockwave-flash" data="http://domain.com/banner.swf" width="150" height="100">
      <param name="movie" value="http://domain.com/banner.swf">
      <param name="wmode" value="opaque"> 
      <param name="flashvars" value="direct_link=true">
      <embed src="http://domain.com/banner.swf" wmode="opaque" flashvars="direct_link=true" width="150" height="100">
   </object>
</div>

In place of 'http://domain.com/banner.swf' give the path of the flash file you want to add.

5. Now save it.