Basically the option for High Availability or load-balancing will appear once you've configured the second link.
The Device > Internet page shows how the appliance connects to the internet.
On this page you can:
Configure a single internet connection or multiple connections in High Availability or Load Balancing configurations. When multiple internet connections are defined, the page shows them in a table.
Will shortly share a screenshot from an appliance I have access to for your reference.
CCSM R77/R80/ELITE