If it's been installed correctly perhaps it was not added to the new admin profiles correctly.

Therefore, you can log in as a Superuser and still see nothing.

I had a similar problem in v1.39h which I assume would happen in v1.51 considering the default admin profile system was changed. Give this a try:
http://www.zen-cart.com/content.php?...e-v1-5-0-admin

I don't have time to test it myself... yet.