Go to the spotlight (magnifying glass in top right) and type Terminal and press Enter.
Within the terminal box type:
'ldapsearch -LLL -x -H ldap://127.0.0.1 -s base namingContexts'
The output should look similar to above, look out for the dc= entries and make a note of it.
Next within the terminal box type:
'sudo ldappasswd -x -H ldapi://%2Fvar%2Frun%2Fldapi -S uid=diradmin,cn=users,dc=macmini,dc=private'
Replace the dc= entries with the entires that are applicable to your setup. Also if the administrators UID isn't diradmin, then change that to what is applicable to your setup.
At the new password prompt, enter a new password and repeat when prompted again.
You then should be able to authenticate within Workgroup Manager.