If I attack and play an ambush unit, Nidalee, Cat Form and my opponent plays nothing and passes focus, do I get another action

Yes — you get focus back.

Playing Nidalee is a card play, not a pass. That play starts a non-initial chain; when it closes, focus moves to the next player in turn order (Rule 347.1.b). Your opponent passing focus is not enough on its own — the Showdown only ends when all players pass in sequence with no card played or ability activated (Rules 347.2.a, 348). So focus comes back to you and you may play another legally timed card or activate an ability.

One nuance worth knowing: the Combat Chain opened from your "when I attack" triggers, so when that initial chain resolves focus stays with you (Rule 346.1). Only after a chain you started closes does focus pass — and it always cycles back once your opponent declines to act.