From this discussion on moodle.org it seems that custom fields are not supported by the standard Moodle LDAP authentication plugin, sorry.

https://moodle.org/mod/forum/discuss.php?d=128903

A patch is in development by Moodle but it is impossible to say how long it will be before it makes it into Moodle core code.

http://tracker.moodle.org/browse/MDL-16982