Both documents are placed on different (sub)domains, by default they are not able to interact via javascript.
You must set the domain of both documents to the same value.
Put this somewhere in the <head/> of both pages:
<script  type="text/javascript">
document.domain='k9webprotection.com'
</script>
...then wait for the onload-event of the iframe and you should be able to access the document inside the iframe from the parent page(and vice versa).
Sample-Script for GreaseMonkey(simply overwrites the body of the iframe):
// ==UserScript==
// @name        k9
// @namespace   k9
// @include     http://www1.k9webprotection.com/get-k9-web-protection-free
// @include     http://license.k9webprotection.com/license.jsp*
// @version     1
// @grant       none
// ==/UserScript==
document.domain= 'k9webprotection.com';
if(self===top){  
  try{
    $('iframe.getk9iframe').load(function(){
       $('body',this.contentDocument)
        .text('gotcha')
          .css({background:'red',fontSize:'3em'});
    });
    alert("I'm the document in the top-window");
  }
  catch(e){}
}