If you want your product accessible from the Cash Register page you can manage this setting from the "Location Settings" section in the Product Set-up page. If the product has already been set up for the Location you are looking at, you can select that location and choose the Cash Register Section to place the product under.

If the product has not been set up yet in a Location, you will need to set the Location, Price, and the Cash Register Section.

There is an option to place this product on any of your Cash Register Sections. It defaults to "Select" which means it will not display.
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If you want to create a new section for this product you will need to go to the Cash Register Admin page.