where do i add orange juice

I feel like the obvious answer is 'in your diary' but perhaps I haven't fully understood your question.
- you go to the diary
- tap on 'add food' under the meal where you want to add your orange juice
- type 'orange juice' in the search bar (or scan the barcode if you have premium)
- choose the entry that corresponds best with the juice you drank
- edit the serving size and/or number of servings
- add to you diary by tapping the checkmark

That's in Android, perhaps a bit different in iOS.