As of v7.4.0, you must set sslEnabled to 1 in the WebGUI conf file for the domain (e.g., /data/WebGUI/etc/YOURSITE.conf).

Once you enable SSL features, the Security tab contains an option "Encrypt content?" which can be used to force the display into https mode.

Note that when you are in Admin On mode, a page that is set to be encrypted can be edited in non-ssl mode. See this thread for more info.