Once you are in the "Products" section of a Quote, Job or Invoice you can select the +Plus icon to add new products.
By choosing the "Lines" option you will be presented with the following popup
This is where you can manually add a single item.
If you have created a catalogue of items you can select the "Retrieve line" button on the top right of the popup.
This will display a list to search and select from
Simply select one of more products to add to the Quote, Job or Invoice you are working on.