substring search problem

[MESSNER D <messner_d@yahoo.fr>]

Hello I want to do a search in a multi valued attribute in that base:

ou=agence

objectclass:agency
o=Lille
employes: uid=dupont,ou=employes,dc=xxx
               uid=dmessner,ou=employes,dc=xxx
               uid=durant,ou=employes,dc=xxx
               uid=henri,ou=employes,dc=xxx


objectclass:agency
o=Marseille
employes: uid=richard,ou=employes,dc=xxx
               uid=paul,ou=employes,dc=xxx
               uid=jack,ou=employes,dc=xxx
               uid=pierre,ou=employes,dc=xxx


I want to search the object with 
(&(objectclass=agency)(employes="uid=dmessner*") and I don't know why it 
doesn't work
It works with (&(objectclass=agency)(o=L*)) but I don't want to find an 
agency by its name but by the employee into it.
Is it a matching rule problem?
The equality for employes is distinguishedNameMatch, is it correct for 
substring search?