The “Log in” link is under “Meta”. Use your Andrew credentials to access the dashboard. Your WordPress site is configured to use Shibboleth, the Single Sign On (SSO) system in use on campus, which eliminates the need for separate accounts on each course site.
Provide access to other Andrew users by creating a user where both the username and email address are listed as the full Andrew email address, e.g., andrewid@andrew.cmu.edu. Andrew logins will not work without this.
If you are giving access to all of your students (as opposed to just instructional staff), you may find the Add Multiple Users plugin helpful.