Where do I type the value of an additional field for one product?

Field values are entered on each product, not on the field itself.

1. Go to Products and open the product to edit it.
2. Click the Additional Fields tab at the top of the product editor.
3. Fill in the fields that appear. Required ones have a red star.
4. Click Save additional fields.

If the tab says no additional fields apply, check the field in Products → Additional Fields. Under Applies to product types, tick this product's type, or leave every box empty so it applies to all types. Fields left empty on a product are simply hidden on the storefront.