It's better to:
*) Show us link of elgg. Although i can search it myself, but if you give the link in post, it will speed up a lot.
*) Show us LDAP authentication related document for elgg, and how you try it and what error/issue you got. Not all users are running Elgg, so you cannot expect someone will show you the detailed step-by-step guide. Instead, show us what error/issue you got while trying to make it work.