I too am having trouble getting an initial test to work. I've tried adding @mydomainname and I've tried reducing the DN to just minimal "DC=A,DC=B,DC=C" (where C is the most general, like com). Still no luck, and my AD LDAP is using standard ports.
Two basic questions that might help me:
1. I don't understand how, if I set my WP site to use SSL for logins and admin, that an LDAP server listening on 389 will understand an encrypted auth request. What am I missing here?
2. Is it clear that this works with WPMU? I'm testing it with the plugin only activated for a subsite.