When I attempt to add my API from Mailchimp, your plugin just keeps saying "Failed - Please check your key and try again."
I've tried creating a brand new API and had the same result. I've used your provided link to access my Mailchimp API dashboard, and that didn't change anything. I am literally copying-and-pasting directly from Mailchimp, so there is no way that I am typing anything wrong.
Why doesn't your plugin work?
ETA: I disabled your plugin, added the free "Mailchimp for WordPress" plugin, pasted my API, and it connected instantly. THAT is how a plugin should work. I just wanted to mention that before you start suggesting that I'm doing something wrong on my end and that's it's not a problem with the plugin.