Declarative definition can be as simple as heading to Shared Components -> Authentication scheme and selecting 'LDAP Directory' from the scheme gallery.
Settings might be as simple as:
Host : myserver
Port : 389
DN String : MYDOMAIN\%LDAP_USER%
Further examples of the distinguished name string are shown in the item help.
|LDAP Authentication definition|
There are a number of other examples in the #orclapex blogosphere on this:
and plenty of support in the OTN forum.
If you're keen to get your hands a little dirtier, there is also a supporting package APEX_LDAP.