...
Go into Settings >> Setup Wizard
Scroll down to the middle of the page where you will see Invoice Items, followed by Click on Products. You will see a table full of your invoiced products.
...
Another option is to filter the available products by typing the product name or code in the search bar.
You can also select multiple products, by either holding ctrl and clicking the products, or clicking the first product, holding shift and clicking the last desired product to select products grouped together. Your selected products will be highlighted.
It is best practice to always press "Save" after you have filled a category to ensure you don't lose the products you just mapped.
...