I assume you're using the Enterprise Edition of the product. You neglect the mention which manual you are referring to.
The SoftwarePlanner User Gude has an entire section on Active Directory Authentication, with screenshots and just about as much detail as I could require. This would be pages 62-67 of SoftwarePlanner Release 9.6.0 User's guide.
Your error logs are found in the DbLogs table of your SoftwarePlanner database. Any LDAP authentication errors will show up there.
You say all you get is error messages. Next time, it would be helpful if you told us what those error messages are! :-)