Now that your brand and Use Case registrations have been accepted, it's time to turn on messaging for your account. Before completing this, you'll be prompted to turn on Multi-factor authentication if it is not presently. Multi-factor authentication in mandatory for all accounts using messaging.

In the Step 3 line, click on Edit.

If Multi-factor Authentication is not enabled, you'll receive this prompt.

Then move the slider to the right to enable messaging and click on Save.  Step 4, adding numbers and users, click here