If you create a guided activity for installing a Sametime 8.5 meeting server you should make sure that you not only can reach you LDAP server via port 389 (resp. 636) but you also can PING it from the system console.
If PING is not allowed e.g. because of a firewall between the system console and the LDAP server you will always get the error message that your LDAP server is not reachable.
If for security reasons normally PING is not allowed you will need to temporarely enable it until your installation has been finished. Afterwards you can disable it again.