''Import LDAP Users by Filter'' does not support sub-tree searches, so you first need to set your Directory Base as already drilled down to the folder that contains the users you want to search for. Check out the following KB article for more details on this feature:
So, once you've set your desired Directory Base via the LDAP Import Settings, you then must use the `(objectClass=*)` method for filtering. For example, let's say you have a user with a common name of Jack Walsh. You could then use `(CN=Jack*)` to find him to add.
***QC/ALM11 has added a powerful ''Import LDAP Users by Keyword'' search option, which doesn't require you to set a Directory Base. It may be time for you to upgrade.*