You'll see that 'index of ...' page when there isn't an index file in the root directory. Installing Zencart places a file named 'index.php' in that directory, so you're doing something wrong.

It isn't clear if you're installing Zencart manually or through some sort of auto-installer your host provides.

The message you're seeing when you try to access your new admin folder only offers two possibilities:

1) You haven't actually installed Zencart to that address.

2) the path information in admin/includes/configure.php is wrong.

I'm thinking it's #1, and you may have actually installed Zencart to a subdirectory, but just in case, you could post the contents of that configure file here after removing the database login details and we can take a look.