It's just plain simple. Hit the + button on the bottom right.
But, there are other ways to get entries into your list. You can paste entries from your clipboard (Open the sidebar with the right top icon).This is very helpful, when you just copy&paste ingredients from a cooking website. You can also just copy&paste entries from one shopping list to another.

Your recipes also do add entries to your shopping list, if you tell the app to do so.

You will be guided to the edit entry screen, where you can change the entry values to your needs.