LDAP module and AD global catalog
I know it's been over half a year so you may have already found the answer to this, but I thought I would weigh in as it is a process that I am working on myself.

For the Base DN, consider simply using DC=mydomain,DC=com.  I have not had to do this yet, so it was just a thought from their suggestion of using a higher Base DN.

I had problems authenticating until I made sure of these items:


AuthName attribute and AccountName attribute set to: sAMAccountName,
PUID set to: objectGuid
then check the box with the question "Does PUID hold a binary value?


After that, authentication went perfectly.  Good luck!