[Dovecot] ACL for root-level mailboxes

Hello:

I see in the documentation where it says that if mailboxes are created at the root level it will use the namespace's default ACL. I'm using Dovecot 2.0.9 so there is no default namespace defined, so I'm a bit handicapped here... Anyway, assuming I can figure out how to define a default namespace, how do I specify its ACL? Where would the file describing the access go?

Sorry if this is a bit broad, but any help would be appreciated.

Thanks,

B. Rintoul