I believe the Holzheimer's site is still 1.3.7 and it works with that. I haven't had a chance to upgrade that site to 1.3.8a yet. If you have Zen Cart unzipped on your computer, you can use something like WinMerge to check the files against one another.

If you do install it, don't forget to give yourself (or whoever'll be the admin for the site) full access to all the functions.