Anyony getting the "could not open default message store" with MAPI and Exchange 2007 installations probably took the default (recommended) option in Exchange Setup to NOT support older MAPI clients. This means there is no "Public Folders" database. Follow the MS KB article on how to create them, basically create a new Public Folder database in your First Storage Group called "Public Folders". Once this was done BES Manager was able to connect without error Enjoy!