Hi,
When you ran the GetAccountHierarchy example, did you make sure that you called SetClientCustomerId on the $user object, passing the manager account? If that doesn't resolve the problem, could you send me the client customer ID of the manager account and the AdWords Express account so I can investigate?
In terms of AdWords accounts vs. AdWords Express accounts, that distinction only exists for the user interface. You can create AdWords Express objects in an AdWords account, and vice versa. However, AdWords Express campaigns cannot be managed via the AdWords API, but they do appear in reports.
Thanks,
Josh, AdWords API Team