If I understand you correctly, you've imported a SWF into Catalyst using Import Artwork on the File menu. When your app runs, you want to pass that swf a parameter that tells it what URL to read a data file from. The SWF is not one that you've created, rather, you got it from some web site.
This is not something that you can do in Catalyst. However it should be possible to do in Flash Builder. I found an article discussing several ways to do it, at http://www.actionscript.org/forums/showthread.php3?t=175951. One idea is the following code snippet
private function setVars(event):void
event.target.content.yyyyyy = "zzzzzz";
<mx:Image source="xxxxx" complete="setVars(event)" />
where "xxxxx" is the path of the SWF (you can put it somewhere in your Flash Builder project, or reference it via an URL), and yyyyy is the name of the parameter that the SWF expects, and zzzzzz is the path to the data file. The <Image> tag is what Catalyst created when you imported the SWF. (btw.the Image and SWFLoader tags are almost identical).
I hope this gives you enough info to make progress.
Thats exactly what i am looking for. Thanks so much for the help.