I asked this same question to the supportdept of Zimbra because the documentation laks proper explanation, this is what one of them came up with:
Then, the zimbra ldap password you need can be retreived on the source machine by executing:
Try with non-ssl and the source ldap should be reachable.
LDAP URL: server-name : 389 : non-ssl
Bind DN: <default>
Bind password: <zimbra_ldap_password of source server>
LDAP filter: <default>
LDAP search base: dc=example,dc=com # example domain: swen.nu
Once you migrate you need add the zimbraMailHost:target server for all newly provisioned users.
Now I had one more question: how to migrate only one mailbox (I have to test migrate first and then I want a step by step migration of users), so I have to modify the LDAP filter... (I just started searching for this, as I stumbled uppon your question...)
su - zimbra -c "zmlocalconfig -s zimbra_ldap_password"