The Stock tab is one of the product tabs which appears with product details in each of these screens: the Catalogues screen, the Search screen or when you view Suppliers > Individual Prices
Open the 
Catalogues window and locate a product using the Search options to access these options on the Stock tab
If the product is not in stock
You will see a shortcut to a Generate Stock Record button.
Click this and you will be taken to the new stock record screen to create the product.
If the product is in stock
You will see an overview of the quantity in stock, quantity allocated to orders and the location of the stocked product.