hi,
while I can hardly say anything about your code, I think you should read up on how to use openssl in objective c (I guess thats what you use to write your app).
The general Idea would be to open an SSL connection to ldapserver port 636 and then through that connection (e.g. the socket), do exactly the same as you did before with an unencrypted connection.
As a side note, LDAPS is considered deprecated in favour of using STARTTLS on Port 389 instead, so you might want to read up on that too.
Regards,