As you upgraded from Zen Cart 1.3.x, your configure.php files will not be altered and will retain the Zen Cart 1.3.x lines for handling the admin folder location. At this time only "clean" installs of Zen Cart 1.5.x will result in the generation of Zen Cart 1.5.x configure.php files with the new features to automatically handle when the admin folder is renamed.
Because your "configure.php" files are from Zen Cart 1.3.x, follow the instructions for Zen Cart 1.3.x in this article:
How Do I rename My Admin.
A useful reference for what each line in the configure.php does can be found in this article:
configure.php file explained.
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