When someone is added as a user, WordPress sends an email with a link to activate their account. The activation code is part of the link and should be automatically activated for WordPress when you click it, but some email clients strip off this functionality for security reasons, so you need to add the Activation Code manually.
If you hover over the link in the email, you'll see the activation key as the last string of characters after the question mark: https://namiaffiliate.org/wp-activate.php?12345678910 Those characters after the ? are the activation key, so in this example the code would be 12345678910
Please let us know if you don't see it, and we can get you set up a different way.