The feature you need to use is Directory Assistance.
Create a directory assistance database on your web server using the provided directory assistance template.
Once the database is created you will create a Directory Assistance document of the “LDAP” type and point it to the LDAP server.
Next select your server document from the All Servers view. From the menu choose Actions->Set Directory Assistance Information. Provide the file name of the directory assistance database you created.
Force the administration process to complete the request using “TELL ADMINP PROCESS INTERVAL”.
Restart the server.
Refer to the following help document for more information:
Lotus Domino Administrator 7 Help – Setting up servers to use a directory assistance database