You can use :

1. SmartWave LDAP Persistence Framework.
See: http://sourceforge.net/projects/swlpf

2. JellyFish (This one is a little bit old, base on Avalon)
See: http://sourceforge.net/projects/jellyfish

I think it's a wise question to ask why we have so much stuff about ORM
but not so much about OHM? Well, I've worked on it for a while, and guess
that the answer is somehow related to the fact that LDAP protocl does not
support TX. Altough the database (for example for OpenLDAP) is
transactional in most servers, but this deficency prevent providers to
make some thing like OHM EntityManager.

Any way, I've developed some sort of general DAO layer based on JNDI and
can be used with various providers such as LDAP or FS. I'm currently
working on it to change it so a more general and open project, but since
then (if the above packages don't help  you) I would be happy to handle
them to you too.