The 'key' value will not be sent to your Common Notification URL, as explained above; it will only be sent to your key product's Payment Variable Information URL when that specific product is purchased. Note that this is NOT necessary simply to deliver a key code to the buyer, as products using the Send Stored/Generated Codes feature will automatically send buyers a thank-you email message providing their purchased key.
It appears your "Social Pro" product is the only one with a Payment Variable Information URL configured for MailChimp, so that is the only product that would send a 'key' value to your MailChimp URL when it is purchased. According to your E-junkie Transaction Log, that product has only ever been purchased once, on April 12th, and we cannot tell whether the product had a Payment Variable Information URL entered at that time; we can only see your current settings. All orders we've processed for you since then have been for a different product, "Social 2 Day Pass", which does not have a MailChimp URL entered in its Payment Variable Information URL.
You would need to contact MailChimp support for any help with settings at their end. We don't actually know much about their integration with us, other than that they receive order data from us via our standard, generic custom/third-party Integration method (as documented at the link provided in my previous reply). I think that their 'ejunkie-ipn' parser normally grabs just the buyer's name and email to subscribe them to a list, so you may need to set up a custom parser with MailChimp to grab and use any other variables you may require.