make a non-hidden one for testing purposes, then navigate to that product in your browser. Look up at your address bar and you'll see the format for how a product URL looks and in that will be an identifier with that product's ID. So that's the format, and then you just have to change the ID to suit the other products. Their ID numbers can be seen in your admin. If you just came to your store, however, keep in mind that a session ID will be tagged on.. be sure not to use that. For example, here's a url from a real store:

http://www.smartieparts.com/shop/index.php?main_page=product_info&products_id=149&zenid=cfb4a133ac5e3192da149ac987953fa3
The grey part is the session ID that you want to delete. It may or may not be in your address bar but if it is, do NOT include it in your links.

The red part is the product ID. That's what you change depending on the product.

Hope that helps. I can't really think of a situation where you'd want a completely hidden shop! And because that is your situation, I have to say that using this mod probably isn't the best way to go about it... but if you find it useful then I wish you the best!

- Steven