OK, this screen doesn't show it too well, but it was the only one I could find.
First I think you have to have your editor in advanced. Do this by clicking on 'Easy' in the top right, and it will switch to 'Advanced' (like in the pic)
See where it says 'Special' under 'Unit'? It's covered up by the unit selection on the pic, but next to that, when creating a unit, it should say 'In formation'. Click on it, and 4 options should come up. 'None', 'In formation', 'In cargo' and 'Flying'. Click on 'flying', and click OK. Then you're ready to go ***