where is bb_admin.php?

I can't find bb_admin.php

In your main folder where all the other .php files are.

NOTE, however, that for security reasons, it may have a different name (if it was setup by your webmaster, for example).

As mentioned by tom322, bb_admin.php should be located in your forum's main folder. If you cannot find a file by that name, it is likely that the file has been renamed, as recommended by the miniBB developers, for better security.
You can single out the renamed bb_admin.php by comparing the files on the web server to the ones listed in the official list of miniBB core files.
All best -
mb

The name of this file should be set under setup_options.php -> $bb_admin

Great question :) Something like - I don't know what I did yesterday :)