If you are having trouble changing the default data file in Outlook 2007 then you are probably trying to change the default file to an account that is synced via IMAP. It is not ever possible to use an IMAP account as the default data file for Outlook since IMAP syncs with the server. If something is sent and/or received from the server then it would also sync to your Outlook 2007 IMAP account. The default data file must be something other than IMAP. One way to do this is to have at least one POP3 account and than you can have as many IMAP accounts after the first account.
I live in the SoBro neighborhood of Louisville, KY with my daughter and our cat Apollo. I enjoy being challenged by technology problems and am fortunate enough to get paid to solve them.