You don't need to configure LDAP on FML to setup personal quarantine on FortiMail.
In your filtering profile(s) just set the action of a failed check (or default action) to PersonalQuarantine and then your FortiMail will send automatically any quarantined mail to the recipient's personal quarantine folder on the FortiMail.
You can then, as admin, either release it manually, or setup FortiMail to send to each recipient a notification mail and allow him (or not) to release the quarantined mail by himself.
As per my experience in general cases you don't need to connect your FortiMail to LDAP, except in few special cases for some special needs.