help : get Element from iframe with bookmarklet

[dedzcnaga]

I'm new in this forumm, newbie in javascript and my English isn't good enough. I hope everyone can understand what I am going to ask.

I am trying to make a bookmarklet for an apps in facebook, which is working with Iframe. The bookmarklet that I want to build is used to change from the apps profile to facebook profile, using the facebook profile id number.

Here is the situation: the frame is the 3rd frame in apps.facebook page, I am working on this 3rd frame. There is a tag like this on the frame :
<div class="tab_content" ><a href="http://xxxxxx?yyy.php?user=zzzzzzzzz" >Profile</a></div>
I want to get that user value "zzzzzzzzz" with bookmarklet, but I don't know how to get that value.
I have tried this, but it didn't work.
javascript:
var el=document.getElementsByTagName("iframe");
var x=el[3].document.getElementsByClassName('tab_content').document.getElementsByTagName('a').);
alert(x);

Please help me with this problem, thanks before