Sorry for the frustration you're having here. We have actually updated our Join My Mailing List feature to no longer use the flow that plugin is using. Existing website signup forms will continue to work but new forms should not be configured with this older technology. The account ID is no longer part of this new version and will not be used in any newly created Join My Mailing List forms. As such, there is actually no longer any way to get the account ID and the outdated instructions this plugin provides are no longer useful.
Our appologies for the frustration with this plugin. I can certainly understand how frustrating it is to find a plugin, install it and then find out that configuring it is incredibly hard. Unfortunately, the developer of this Wordpress widget did not work with us in developing and releasing this plugin and the developer is also no longer maintaining it. We also do not have any ability to remove this plugin as we do not own it.
We will continue to reach out to Wordpress.org to see if we can get this plugin removed. In the short term, we have no plans to provide this account ID externally. This ID has only ever been intended to be used internally by Constant Contact systems.
As Dave had mentioned in his post, we are aiming to use methods of integrating with Constant Contact accounts that do not involve your account ID. This ID is intended to be internal and we aim to maintain that as much as possible. Our updated APIs and systems are able to provide a greater level of functionality while keeping this information internal to Constant Contact.
Okay, I ended up using the Module for Joomla called Nice Constant Contact and their instructions were a bit clearer on where to locate the ID.
1. Login to your Constant Contact account.
2. Go to Contacts List.
3. Click on the Tab 'ADD CONTACTS' and then click on the drop down for GET NEW CONTACTS
4. Click the Create a link symbol to the right under ACTIONS.
5. Click SHARE URL to FORM.
6. Now you can view the "HTML Code".
7. In the HTML code find the line where name="m", see below. The value stored in this line is the account id. So, in our example below, YOUR_CODE_IS_HERE is the account id.
8. Enter the retrieved account id number in your Constant Contact Signup Module parameters and the form will work.
Hope this helps.