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 look at the screenshot of a new user invitation email below, the activation key is the numbers in red on the URL - in this case - 03a17370c059789f
Please let us know if you don't see it, and we can get you set up a different way.