Welcome to the new file system!!! Backups (.mbz's now) are spread over moodledata/filedir/ in sub directories and filename that have been hashed ... no longer humanly recognizable. The 'humanly recognizable' names of those backup files you might see in the Moodle UI is really generated by the DB now and the DB knows the contenthash given all files.
A little bit more information would have been helpful as Bitnami can be installed on Linux (locally hosted or remotely hosted), Windows and MacOSX. So the following is related to Linux based OS ... similar to MacOSX and dis-similar to Windows.
The designated directory (alternative) you created in /etc/ is probably restricted ... ie, apache user cannot write to it.
Bitnami is one of those 'easy button' thangs (or is supposed to be). While it might be easy to install, that's just tip of the iceburg when it comes to Moodle (there are, as you are experiencing now) server administration duties .. like backups of courses, backups of DB (by file or SQL dump), backups of code directories (which, in Bitnami is where?), and backups of the data directory.
So here are some references for Bitnami:
http://wiki.bitnami.com/Components/Apache
http://wiki.bitnami.com/Components/MySQL
http://wiki.bitnami.com/Components/PHP
http://wiki.bitnami.com/Components/PHP_Frameworks/Zend_Framework
Those, should you continue to use Bitnami, might be your primary resources for your system.
Back to your alternative directory issue ... that directory could be located anywhere on the system really, but to assure it works one must allow the user under which apache runs at least write access to it. Would advise that /etc is not a good location for alternate location Moodle backups. Do you have a home directory (for the user you login under)? - like /home/mspit You could create a 'moodlebackups' in home and set ownerships/permissions to it where apache could write to it: chown apache:apache /home/moodlebackups/
The alternative directory will not be seen in the Moodle UI ... it's purpose, I think, is strictly archival.
Hope that helps!
'spirit of sharing', Ken