IPN arrrggggggg!

Hi there, I've been trying to set up the IPN, following the steps outlined in this post:
https://premium.wpmudev.org/forums/topic/multiples-ipn-dynamically-setting-the-notification-url

I must be doing something wrong and I can't figure out what it is.

We have an MU install + pro sites. I downloaded the ipn-forwarder.php from the previous post and edited this part, I'm only using it on this one url:

//Our password. Check for this in your script to make sure it's from us ($_POST['inc_pass']). Do not change!
define('INC_PASS', 'thepasswordImadeup');

// An array of prefix to search for, domain and path to post to, and whether we're using sandbox or not for applications
$apps = array(
  /* App 1 */
  array('prefix' => 'trailsherpa',
        'domain' => 'trailsherpa.com',
        'path'   => '/wp-admin/admin-ajax.php?action=psts_pypl_ipn',
        'live'   => true),

);

then this is in the wp-config:

/* paypal IPN */
define('PSTS_IPN_PASSWORD', 'thepasswordImadeup');
define('PSTS_PYPL_PREFIX', 'trailsherpa');
/* That's all, stop editing! Happy blogging. */

then in my paypal IPN page I have this:

Notification URL http://www.trailsherpa.com/ipn-forwarder.php (this is correct btw)
Message delivery Enabled

when I test that url in IPN testing it says that it goes thru, but the IPN history page shows no transactions.

So I tried turning the paypal on live and did a real transaction (a new site upgrade) and it looked like it went thru but on the IPN history page I was getting a retrying message:

Date/time created Message ID Status Transaction ID
2/21/2012 12:53 PST 9VH87738HM698224L Retrying
2/21/2012 12:53 PST 3GX88566NB5324742 Retrying

If i go to this url: http://www.trailsherpa.com/ipn-forwarder.php

in a browser I get this message: Error: Missing POST variables. Identification is not possible. which is supposedly a good sign...