Download the virtual box platform installer from here: https://www.virtualbox.org/wiki/Downloads
Once it has finished downloading, double click on the dmg file and follow the instructions (number 1) for installing. it should look like this:
follow the subsequent steps by clicking 'continue' and 'install'
Download from here. Please note this is a large file (~1.8GB) and may take some time.
Click on the ova file, this should open in VirtualBox. Then import the virtual machine by clicking the import button.
That's it, you will now be able to run the machine, and will be able to sync any updates on the morning of the workshop.