Go to Configure , Certificate Go to Add a trusted CA and import "ca_bundle.crt". Go to Import Signed Certificate and import "certificate.crt". if certificate.crt is fine, you will be asked to import the "private.key" and reboot.
If you are using 2 ZoneDirector for HA, then you need to do it in both sides.
Problem is that you have to make and install new certificate every 3 month, as certificate expiration is 90 days -- so it doesn't make much sense for me... Easier to pay once a 3 years for certificate and forget about it until next time. Hope it helps