Cocooningweb, I see a couple issues based on your description:
The Windows Browser Host should not be the Domain, but instead is the computer in your network from which Mobile File Manager fetches a list of available domains and computers. Do not use a Domain Controller host name. Instead, use another host such as your BlackBerry Enterprise Server, Exchange server or a file server.
You should not need to use domain\username for the username as the domain that the computer is on will be automatically used.