I added inventory items to my site on the backend, but I cannot see them listed on the inventory window on the site itself?

A B2B site can be setup so that a Site Group can access inventory. This involves some modifications to the site groups and site navigation as documented here. Once that setup is done, an Inventory menu item will be available on the site, that takes the site user to a window that shows the active inventory items on that site.
An active inventory item is one that is assigned to a product. Inventory items that are not assigned to any products will not show on the Inventory window listing for a B2B site when being viewed by a user who can see the inventory. The inventory items you create in Pressero admin when configuring the site must be actively assigned to a product before they will be available on the site front-end inventory report window.  In order for the user to see this item within a report, they must be within an active Site Group that has the proper permission to see that product.