The quickest way to add an individual item is to use the search field New item in the toolbar directly above the input window. Enter a search term such as name, item number or PLU number and press Enter to confirm.
Open the standard item search by clicking on the dropdown triangle next to Add and select Search items.
Select one item by left-clicking (hold down the Ctrl key for multi-select) and click on Apply to copy the items you want.
To insert all items of one item group collectively, select the group in the item search, mark all items in the list of results (CTRL+A) and click on Apply.