First, download the certificate file from
cert.carmichael.qld.edu.au
After downloading, find and click (or double-click) the certificate to install it.
If it asks you to enter a password, please enter the password you use to sign in to your computer.
Now that the certificate is in your Keychain, you need to find it and trust it. It should be called "CMC-Trust-CA".
Once you find it, double-click it, click on trust, and set "When using this certificate:" to "Always Trust"
After trusting the certificate, click the X to close the window and enter your password again if prompted.
To make sure you are online, try visiting
cats.com