HALP. Can't get editor body content. AJAX related.

Here's my editor code:

include("../editor/fckeditor.php");
$oFCKeditor=new FCKeditor('userSignature');
$oFCKeditor->BasePath='system/editor/';
$oFCKeditor->Create();

Which is inside a form called:

<form name='updateSignature'>

But i don't have a 'form action', instead;
On preview button 'onclick', this is done:

var userSignature=document.getElementById('userSignature').value;
xmlHttp.send("cmd=previewsignature&sig="+userSignature);

That's cause i don't want page to refresh, it's AJAX, and form submit would make page refresh.

The problem is that i can't get the value of the editor. Whatever is typed in the editor, i simply can't put into the userSignature var. Maybe somebody here knows what i'm doing wrong. I sincerely hope so.