The Comments 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 Comments tab
You have 3 types of comments:
User Comments
Added by the user, and will show on screen when users add a product to a quote/order.
Product Comments
These are added within the EQ catalogue, and are stored uniquely against each product. The comments will be displayed when you save a quotation/order on the validation rules screen. Note you may have the validatation rule disabled, check in Tools > Validation Rules.
Supplier Comments
These are added within the EQ catalogue, and are stored uniquely against each supplier. The comments will be displayed when you save a quotation/order on the validation rules screen. Note you may have the validatation rule disabled, check in Tools > Validation Rules.