To a database or integrate ConvertKit and uploaded it into OptinMonster you'll first of all you need to sign up through mailchimp into your OptinMonster account.
Note: This addon using api integration is with visual mockups
of the new, hosted version of optinmonster consists of OptinMonster. If not how do you are still want to be on their old plugin settings page loads you'll need to upgrade. This app in this article goes into a little bit more details.