You cannot use this effect to play Dryad Arbor during an opponent's turn.

You don't choose a creature card when Scout's Warning resolves. Rather, this sets up a rule that is true for you until the turn ends or until you cast a creature card, even if you cast that creature at a time you normally could.

After Scout's Warning resolves, you can suspend a creature card in your hand any time you can cast an instant. As soon as you actually cast a creature card, you lose this capability.

If you cast multiple Scout's Warnings on the same turn, they'll all apply to the very next creature spell you cast.