When Mimecast wants you to enroll the device you are working from it will open a webpage requesting your email.

Enter your email and click, Get Authentication Code.  DO NOT CLOSE THIS WEBPAGE.

Check your email for the Enrollment Code.

Copy or remember the code.

Go back to the webpage that originally requested your email address.  It is now waiting for your Enrollment Code.

Enter your Enrollment Code and click Enroll Device.

If you did not receive a code, click Start Again and the process will start over.