Hi Joe!
What's there looks good to me! From what I can see there, all you should need to do is upgrade your account. You're currently over the limit for the level you are on. Mimi returns a code and a short explanation on why a user is not added. In this case it should be something like this:
Mimi allows members to be added over the limit by webform provided at
madmimi.com/signups, but users cannot be added over the limit via API, and nor can they be sent to by any means until the upgrade is done.
Best,
Sally L