Open iim.conf.
See iim.conf File Syntax for instructions on locating and modifying iim.conf.
Specify the attribute you want to use to display usernames as the value for iim_ldap.userdisplay.
For example, to use the nickname attribute, set the iim_ldap.userdisplay attribute as follows:
iim_ldap.userdisplay=nickname
Save and close the file.