Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
I’m going to escalate this case to one of our Tier 2 support specialists so that they can follow up directly regarding the issue you're experiencing while attempting to connect the Constant Contact Forms for Wordpress plugin. They should be in touch via the contact info we have on file for your account within 4 business days. Otherwise, for immediate assistance, or if you’d like a callback/email from them at a certain day/time you can call our support line at 866-289-2101 and reference case #30057538.
Please have a look and let us know if you have any other questions!