first you should consult http://support.apple.com/kb/HT4837
and then you have to import the specific users. You can do this in the Server.app by adding a new user an then select not new user but instead import the user from the Active Directory. I hope this helps. It is confusing
that Lion Server does not uses the Active Directory to store the information, but instead creates a new Open Directory Master an uses augumented entries for the ADS users.
Ok I have also found another way for those that are interested that allows access to all active directory users rather than adding to the profile manager on a per user basis.
1) install the Lion Server tools
2) under the access icon select the profile manager service in the LH column and in the RH column select All users and groups
Alternately you can drag users directly to the RH column for those you want to have access rather than adding them as a local user