We will fix the FTP issue in the next version, currently you can create a password for the bitnami user in hte machine and specify it in the FTP configuration. We will automate this process in next releases:
$ sudo passwd bitnami
You can find more info at http://wiki.bitnami.com/Applications/BitNami_Wordpress_Stack#How_to_install_a_plugin_on_Wordpress.3f
If you have changed the URL it is necessary to update the Wordpress database. You can use the "updateip" tool to change it.
$ sudo /opt/bitnami/updateip --machine_hostname NEW_HOSTNAME_OR_IP
This tool is executed every reboot to update the domane name of the machine (ec2-xx-xx-xx-amazonaws.com). If you have set your own domain you can rename it to not update your IP automatically.
I hope it helps.