• Don’t try to run Helene directly (that is, without the <iframe> indirection). I thought it would be easier and the integration would be more seamless but that wasn’t the case
• Helene has to run on the same server as your script. That is, because helene runs in the <iframe>, you have to write to the editor (that is: textarea) after reading a file and reading from the editor when posting the form (that is, you do that via onsubmit parameter of <form>). If helene runs on a different server (that is, the src-parameter of <iframe> points to a different server), javascript suppresses that for security reasons
• Read the HOWTO of helene. It spares you some headache (it deals with removing some newline characters from the input file)
• Don’t try to use <object> instead of <iframe>. I don’t remember exactly but I think ie had problems with <object>
• After reading from $_POST I had to
  
$source = $_POST['helene']
$source = stripslashes($source);
$source = str_replace("\r\n", "\n", $source);


That’s it. I hope it helps someone. I surely hope that someone does some further improvements on this great editor.