HeleneWallis:
One other thing, though--the directions specify three core files in the admin section, but I only see two--footer.php and stylesheet.css. What other file is supposed to be merged?
Whoops, you replied faster than I expected. I'm using a minor modification of the classic template.
That's a documentation error; the change to the admin's index.php (also a core file) is now conditional (based on the 1.5.5 admin changes) and only affect the superglobals' display on the admin front-page.
The plugin uses Zen Cart 1.5.4 as its core-file basis; there's one line
@import url("stylesheet_superglobals.css");/* including stylesheet for superglobals display */
that needs to be added to the admin's stylesheet.css (after the other @import statements) and one code block
<!--bof- superglobals display -->
<?php
// BEGIN Superglobals
if(SHOW_SUPERGLOBALS_ADMIN == 'true') echo superglobals_echo();
// END Superglobals
?>
<!--eof- superglobals display -->
that needs to be added at the very end of the admin's footer.php.