MacOS Sierra (10.12)
I can't supply any screenshots I'm afraid as I have already wasted too much time on this. After 5 installs I have been forced to give up on the Bitnami package and have just downloaded ModX directly. I installed that with no problems and it works fine.
During that install I was indeed prompted for an Id and Password for the Admin. The only "hint" I can give you on the cause of my problems is that during the Bitnami install there is a dialog that asks for the MySQL root password. When it ass that question the "Next" button is visible gut greyed out. Once the password has been entered and the database creation process has run, that button disappears and the dialog simply says that the install has been completed. This is obviously the point at which the problem occurs because when I ran ModX's own install this was the point at which I was asked for the Admin Id and password. It would appear that your script is deciding for some reason that that question does not need to be asked and is skipping it.
Thanks for trying anyway.