Jim, sorry to post with a problem that should be easy, but I think I've read through this entire topic as well as the PDF, and just can't find the solution!.
Brand new installation of osCommerce, 2.3.3
Downloaded Theme Switcher 1.4.2 min
Copied ht_theme_switcher.php files into respective folders, and template_top.php into catalog/includes folders. Checked many times that the ht_theme_switcher.php files are not reversed.
When I go to the modules panel and select Header Tags, the 'Install Module' button shows 4 modules available, but when I click it, only three modules are listed -- Theme Switcher isn't there!
If I've missed something, I just can't find it...