According to the instructions for the Fix Cache Key mod, you should put the file "fix_cache_key.php" file where the "index.php" for your Zen Cart store is located.

For your case, it would mean putting the file into the document root of your server since your Zen Cart store is not located in a sub-directory.

From the error message you've pasted here, it seems like you placed that file in the wrong directory. Try fixing that and see if things work.