Hey Zig,
Usually when you receive an email from email address a, and reply, it will default to reply using that account (email address a), same for email address b, c, etc... When you compose a new message from the main inbox, you will have an option about the To: field that says "Send Using: ...." That is where you can specify what account. For me I have Desktop (my work email via BES), Yahoo, MSN & vzw.blackberry.net.
The other option is to go into the mail file specific inbox, these should have been added by the BIS for each account. When you compose a mail from within them, it will default to that mail address.
Hope this helps and have fun